Only disable -Wthread-safety-attributes warning for Clang.

Clang is enabled by default and we currently lack coverage for building on Windows
with MSVC (see crbug.com/757293). This should unblock rolling WebRTC into
Chromium DEPS. We need to improve our trybot coverage for standalone WebRTC
to prevent things like this in the future though (crbug.com/756840).

BUG=webrtc:8119,chromium:756840,chromium:757293
TBR=hbos@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/3003473002
Cr-Commit-Position: refs/heads/master@{#19420}

webrtc/modules/video_coding/BUILD.gn

@@ -148,7 +148,9 @@ rtc_static_library("video_coding_utility") {
 
 # TODO(ehmaldonado): Remove once http://bugs.webrtc.org/8090 is fixed.
 config("webrtc_h264_config") {
-  cflags = [ "-Wno-thread-safety-attributes" ]
+  if (is_clang) {
+    cflags = [ "-Wno-thread-safety-attributes" ]
+  }
 }
 
 rtc_static_library("webrtc_h264") {