Make media::WallClockTimeSource thread safe.

Temporary measure until both Pipeline is reworked to provide a time
estimate to the main thread and VideoRendererImpl's internal thread
is eliminated.

This is similar to the the locking currently present in
AudioRendererImpl.

BUG=412764

Review URL: https://codereview.chromium.org/562673003

Cr-Original-Commit-Position: refs/heads/master@{#294207}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 465cf3e643d44ce7176072bdbac1a9b03ac70514

sanitizers/tsan_suppressions.cc

@@ -301,9 +301,6 @@ char kTSanDefaultSuppressions[] =
 "deadlock:"
 "base::debug::TraceEventTestFixture_ThreadOnceBlocking_Test::TestBody\n"
 
-// http://crbug.com/412764
-"race:media::WallClockTimeSource::SetPlaybackRate\n"
-
 // End of suppressions.
 ;  // Please keep this semicolon.