cc: Add mailbox support to ResourceProvider write locks.

cc/raster/raster_buffer_provider.h

Index: cc/raster/raster_buffer_provider.h
diff --git a/cc/raster/raster_buffer_provider.h b/cc/raster/raster_buffer_provider.h
index 061740a0e06eb4428f46232be6b618b2f38d7be2..040585c0dd0e2e91327a2986e17dbe1df766224b 100644
--- a/cc/raster/raster_buffer_provider.h
+++ b/cc/raster/raster_buffer_provider.h
@@ -53,8 +53,9 @@ class CC_EXPORT RasterBufferProvider {
   // Release raster buffer.
   virtual void ReleaseBufferForRaster(std::unique_ptr<RasterBuffer> buffer) = 0;
 
-  // Barrier to sync resources to the worker context.
-  virtual void OrderingBarrier() = 0;
+  // Used for syncing resources to the worker context. Returns false if barrier
+  // could not be established.
+  bool OrderingBarrier() = 0;
 
   // Returns the format to use for the tiles.
   virtual ResourceFormat GetResourceFormat(bool must_support_alpha) const = 0;