Make CommandBuffer::Flush asynchronous, and add CommandBuffer::FlushSync with former semantics.

gpu/command_buffer/client/cmd_buffer_helper.h

Index: gpu/command_buffer/client/cmd_buffer_helper.h
diff --git a/gpu/command_buffer/client/cmd_buffer_helper.h b/gpu/command_buffer/client/cmd_buffer_helper.h
index d174831ec95d19043e976a022577ff4813515254..7e6fcfccb077ac54e90bf1f6b084d1734eec6f5f 100644
--- a/gpu/command_buffer/client/cmd_buffer_helper.h
+++ b/gpu/command_buffer/client/cmd_buffer_helper.h
@@ -48,7 +48,8 @@ class CommandBufferHelper {
   // have made some progress in processing them. Returns whether the flush was
   // successful. The flush will fail if the command buffer service has
   // disconnected.
-  bool Flush();
+  bool FlushSync();
+  void Flush();
 
   // Waits until all the commands have been executed. Returns whether it
   // was successful. The function will fail if the command buffer service has