Suppress a debug-only race in SkBaseMutex::acquire().

BUG=skia:3294
TBR=bungeman@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/791163003

Cr-Commit-Position: refs/heads/master@{#310696}

build/sanitizers/tsan_suppressions.cc

@@ -306,6 +306,9 @@ char kTSanDefaultSuppressions[] =
 // Suppressing both AudioContext.{cpp,h}.
 "race:modules/webaudio/AudioContext\n"
 
+// https://code.google.com/p/skia/issues/detail?id=3294
+"race:SkBaseMutex::acquire\n"
+
 // End of suppressions.
 ;  // Please keep this semicolon.