cc: Add mailbox support to ResourceProvider write locks.

cc/raster/zero_copy_raster_buffer_provider.cc

Index: cc/raster/zero_copy_raster_buffer_provider.cc
diff --git a/cc/raster/zero_copy_raster_buffer_provider.cc b/cc/raster/zero_copy_raster_buffer_provider.cc
index 93e8d7e13f07440d2bb6982b62466a5f0cc436d4..c6e43ee072f0a7afe17489ee6db303e3c9664aca 100644
--- a/cc/raster/zero_copy_raster_buffer_provider.cc
+++ b/cc/raster/zero_copy_raster_buffer_provider.cc
@@ -55,6 +55,8 @@ class RasterBufferImpl : public RasterBuffer {
     buffer->Unmap();
   }
 
+  void SetSyncToken(const gpu::SyncToken& sync_token) override { NOTREACHED(); }
+
  private:
   ResourceProvider::ScopedWriteLockGpuMemoryBuffer lock_;
   const Resource* resource_;