Revert "Transport sequence number should be set also for retransmissions."

After this CL, video_engine_test started failing flakily in different bots for different CLs.

TBR=sprang@webrtc.org

Review URL: https://codereview.webrtc.org/1393553003 .

Cr-Commit-Position: refs/heads/master@{#10188}
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
index 9d134af..252ffb2 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
@@ -695,7 +695,6 @@
   size_t length = IP_PACKET_SIZE;
   uint8_t data_buffer[IP_PACKET_SIZE];
   int64_t capture_time_ms;
-
   if (!packet_history_.GetPacketAndSetSendTime(packet_id, min_resend_time, true,
                                                data_buffer, &length,
                                                &capture_time_ms)) {
@@ -923,8 +922,8 @@
   // TODO(sprang): Potentially too much overhead in IsRegistered()?
   bool using_transport_seq = rtp_header_extension_map_.IsRegistered(
                                  kRtpExtensionTransportSequenceNumber) &&
-                             transport_sequence_number_allocator_;
-
+                             transport_sequence_number_allocator_ &&
+                             !is_retransmit;
   PacketOptions options;
   if (using_transport_seq) {
     options.packet_id =
diff --git a/webrtc/video/end_to_end_tests.cc b/webrtc/video/end_to_end_tests.cc
index 715399a..305dcb0 100644
--- a/webrtc/video/end_to_end_tests.cc
+++ b/webrtc/video/end_to_end_tests.cc
@@ -20,7 +20,6 @@
 #include "webrtc/call.h"
 #include "webrtc/call/transport_adapter.h"
 #include "webrtc/frame_callback.h"
-#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_utility.h"
 #include "webrtc/modules/video_coding/codecs/vp8/include/vp8.h"
 #include "webrtc/modules/video_coding/codecs/vp9/include/vp9.h"
@@ -1336,6 +1335,9 @@
 }
 
 TEST_F(EndToEndTest, AssignsTransportSequenceNumbers) {
+  // TODO(sprang): Extend this to verify received values once send-side BWE
+  // is in place.
+
   static const int kExtensionId = 5;
 
   class RtpExtensionHeaderObserver : public test::DirectTransport {
@@ -1345,8 +1347,7 @@
           parser_(RtpHeaderParser::Create()),
           last_seq_(0),
           padding_observed_(false),
-          rtx_padding_observed_(false),
-          retransmit_observed_(false) {
+          rtx_padding_observed_(false) {
       parser_->RegisterRtpHeaderExtension(kRtpExtensionTransportSequenceNumber,
                                           kExtensionId);
     }
@@ -1360,8 +1361,6 @@
 
       RTPHeader header;
       EXPECT_TRUE(parser_->Parse(data, length, &header));
-      bool drop_packet = false;
-
       if (header.extension.hasTransportSequenceNumber) {
         EXPECT_EQ(options.packet_id,
                   header.extension.transportSequenceNumber);
@@ -1371,25 +1370,12 @@
         }
         last_seq_ = header.extension.transportSequenceNumber;
 
-        // Drop every 20th packet, so we get retransmits.
-        if (header.sequenceNumber % 20 == 0) {
-          dropped_seq_.insert(header.sequenceNumber);
-          drop_packet = true;
-        }
-
         size_t payload_length =
             length - (header.headerLength + header.paddingLength);
         if (payload_length == 0) {
           padding_observed_ = true;
         } else if (header.payloadType == kSendRtxPayloadType) {
-          uint16_t original_sequence_number =
-              ByteReader<uint16_t>::ReadBigEndian(&data[header.headerLength]);
-          if (dropped_seq_.find(original_sequence_number) !=
-              dropped_seq_.end()) {
-            retransmit_observed_ = true;
-          } else {
-            rtx_padding_observed_ = true;
-          }
+          rtx_padding_observed_ = true;
         } else {
           streams_observed_.insert(header.ssrc);
         }
@@ -1397,14 +1383,12 @@
         if (IsDone())
           done_->Set();
       }
-      if (drop_packet)
-        return true;
       return test::DirectTransport::SendRtp(data, length, options);
     }
 
     bool IsDone() {
       return streams_observed_.size() == MultiStreamTest::kNumStreams &&
-             padding_observed_ && retransmit_observed_ && rtx_padding_observed_;
+             padding_observed_ && rtx_padding_observed_;
     }
 
     EventTypeWrapper Wait() { return done_->Wait(kDefaultTimeoutMs); }
@@ -1413,10 +1397,8 @@
     rtc::scoped_ptr<RtpHeaderParser> parser_;
     uint16_t last_seq_;
     std::set<uint32_t> streams_observed_;
-    std::set<uint16_t> dropped_seq_;
     bool padding_observed_;
     bool rtx_padding_observed_;
-    bool retransmit_observed_;
   };
 
   class TransportSequenceNumberTester : public MultiStreamTest {