Suppress races in vp3_update_thread_context

This was uncovered when we removed the bulk ffmpeg suppressions. If
ClusterFuzz comes back and reports tons of different threading issues,
we probably need to go back to blanket suppressing everything. For now,
see if we can target something more fine-grained this time.

Bug: 977085
Change-Id: If19c254e0b98c5d450b74cc7d02fd7a2f1a0e189
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669760
Commit-Queue: David Benjamin <davidben@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671095}

build/sanitizers/tsan_suppressions.cc

@@ -148,6 +148,9 @@ char kTSanDefaultSuppressions[] =
     // https://crbug.com/965722
     "race:content::(anonymous namespace)::CorruptDBRequestHandler\n"
 
+    // https://crbug.com/977085
+    "race:vp3_update_thread_context\n"
+
     // End of suppressions.
     ;  // Please keep this semicolon.