cc: Add message passing mechanism to micro benchmarking.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 793cd69c253e782ea1cc8d44553ff1755dda0591..9d9c5d106207e93a1165c56468c4cb7fb3603142 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -275,9 +275,12 @@ class CC_EXPORT LayerTreeHost {
   bool UsingSharedMemoryResources();
   int id() const { return id_; }
 
-  bool ScheduleMicroBenchmark(const std::string& benchmark_name,
-                              scoped_ptr<base::Value> value,
-                              const MicroBenchmark::DoneCallback& callback);
+  // Returns the id of the benchmark on success, 0 otherwise.
+  int ScheduleMicroBenchmark(const std::string& benchmark_name,
+                             scoped_ptr<base::Value> value,
+                             const MicroBenchmark::DoneCallback& callback);
+  // Returns true if the message was successfully delivered and handled.
+  bool SendMessageToMicroBenchmark(int id, scoped_ptr<base::Value> value);
 
   // When a SwapPromiseMonitor is created on the main thread, it calls
   // InsertSwapPromiseMonitor() to register itself with LayerTreeHost.