clang/win: Fix -Wextra warnings in webrtc.

Fixes one sign mismatch warning, and one "const has no effect and is
ignored" warning.

BUG=chromium:567877

Review URL: https://codereview.webrtc.org/1510233002

Cr-Commit-Position: refs/heads/master@{#10976}
diff --git a/webrtc/base/sec_buffer.h b/webrtc/base/sec_buffer.h
index d4cda00..e6ffea4 100644
--- a/webrtc/base/sec_buffer.h
+++ b/webrtc/base/sec_buffer.h
@@ -119,7 +119,7 @@
   }
 
   // Accessor for the descriptor
-  const PSecBufferDesc desc() const {
+  PSecBufferDesc desc() const {
     return &desc_;
   }
 
diff --git a/webrtc/base/systeminfo.cc b/webrtc/base/systeminfo.cc
index bfc96b3..b400aa0 100644
--- a/webrtc/base/systeminfo.cc
+++ b/webrtc/base/systeminfo.cc
@@ -110,7 +110,7 @@
   DWORD_PTR process_mask = 0;
   DWORD_PTR system_mask = 0;
   ::GetProcessAffinityMask(::GetCurrentProcess(), &process_mask, &system_mask);
-  for (int i = 0; i < sizeof(DWORD_PTR) * 8; ++i) {
+  for (size_t i = 0; i < sizeof(DWORD_PTR) * 8; ++i) {
     if (process_mask & 1)
       ++cur_cpus;
     process_mask >>= 1;
diff --git a/webrtc/modules/desktop_capture/win/screen_capturer_win_magnifier.cc b/webrtc/modules/desktop_capture/win/screen_capturer_win_magnifier.cc
index db40478..b5eb1c0 100644
--- a/webrtc/modules/desktop_capture/win/screen_capturer_win_magnifier.cc
+++ b/webrtc/modules/desktop_capture/win/screen_capturer_win_magnifier.cc
@@ -236,7 +236,7 @@
     RECT unclipped,
     RECT clipped,
     HRGN dirty) {
-  assert(tls_index_.Value() != TLS_OUT_OF_INDEXES);
+  assert(tls_index_.Value() != static_cast<int32_t>(TLS_OUT_OF_INDEXES));
 
   ScreenCapturerWinMagnifier* owner =
       reinterpret_cast<ScreenCapturerWinMagnifier*>(
@@ -369,7 +369,7 @@
     }
   }
 
-  if (tls_index_.Value() == TLS_OUT_OF_INDEXES) {
+  if (tls_index_.Value() == static_cast<int32_t>(TLS_OUT_OF_INDEXES)) {
     // More than one threads may get here at the same time, but only one will
     // write to tls_index_ using CompareExchange.
     DWORD new_tls_index = TlsAlloc();
@@ -377,7 +377,7 @@
       TlsFree(new_tls_index);
   }
 
-  assert(tls_index_.Value() != TLS_OUT_OF_INDEXES);
+  assert(tls_index_.Value() != static_cast<int32_t>(TLS_OUT_OF_INDEXES));
   TlsSetValue(tls_index_.Value(), this);
 
   magnifier_initialized_ = true;