Suppress race to cache SkPathRef bounds.

This should be fixed naturally soon, and it's an innocuous race (some paths
may have their bounds calculated more than once), so we can just quiet it for now.

BUG=skia:
R=mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/376733003

tools/tsan.supp

@@ -33,3 +33,9 @@ race:SkPixelRef::needsNewGenID
 
 # This calls SkRefCnt::getRefCnt(), which is not thread safe.  skia:2726
 race:SkImageFilter::filterImage
+
+# SkPathRef caches its bounding box the first time it's needed.
+# This will be fixed naturally once we create (from a single thread) a
+# bounding-box hierarchy for SkRecord-based SkPictures; all bounds will come pre-cached.
+# So just shut this up for now.
+race:SkPathRef::computeBounds