Add MGLEcho

gpu/command_buffer/client/gpu_control.h

Index: gpu/command_buffer/client/gpu_control.h
diff --git a/gpu/command_buffer/client/gpu_control.h b/gpu/command_buffer/client/gpu_control.h
index 2b303b3b4d58ced50269d00d2598e29c248dcf4d..dce809d8c938d032460f25a25b2f79c54228bdf3 100644
--- a/gpu/command_buffer/client/gpu_control.h
+++ b/gpu/command_buffer/client/gpu_control.h
@@ -52,6 +52,8 @@ class GPU_EXPORT GpuControl {
                                              unsigned internalformat,
                                              unsigned usage) = 0;
 
+  virtual void Echo(const base::Closure& callback) = 0;
+
   // Inserts a sync point, returning its ID. Sync point IDs are global and can
   // be used for cross-context synchronization.
   virtual uint32_t InsertSyncPoint() = 0;