Fix implicit size_t to uint16_t warning on VS2015.

Reviewed as https://review.webrtc.org/54609004/, but uploaded against
wrong Base URL.

R=pbos@webrtc.org
TBR=
BUG=chromium:440500

Review URL: https://webrtc-codereview.appspot.com/54649004

Patch from scottmg <scottmg@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#9351}
diff --git a/webrtc/video/video_send_stream.cc b/webrtc/video/video_send_stream.cc
index 254b7d6..45ea466 100644
--- a/webrtc/video/video_send_stream.cc
+++ b/webrtc/video/video_send_stream.cc
@@ -177,8 +177,8 @@
   vie_capturer_ = new ViECapturer(module_process_thread_, vie_encoder_);
 
   // 28 to match packet overhead in ModuleRtpRtcpImpl.
-  vie_channel_->SetMTU(
-      static_cast<unsigned int>(config_.rtp.max_packet_size + 28));
+  DCHECK_LE(config_.rtp.max_packet_size, static_cast<size_t>(0xFFFF - 28));
+  vie_channel_->SetMTU(static_cast<uint16_t>(config_.rtp.max_packet_size + 28));
 
   DCHECK(config.encoder_settings.encoder != nullptr);
   DCHECK_GE(config.encoder_settings.payload_type, 0);