implement getBufferSubDataAsync prototype

gpu/command_buffer/client/gles2_interface.h

Index: gpu/command_buffer/client/gles2_interface.h
diff --git a/gpu/command_buffer/client/gles2_interface.h b/gpu/command_buffer/client/gles2_interface.h
index 0b0bc514f2489822582482a66fa8806237cd5e12..0457d5eda1a596612d20841ad35a47943373edee 100644
--- a/gpu/command_buffer/client/gles2_interface.h
+++ b/gpu/command_buffer/client/gles2_interface.h
@@ -20,6 +20,8 @@ class GLES2Interface {
   GLES2Interface() {}
   virtual ~GLES2Interface() {}
 
+  virtual void FreeSharedMemory(void*) {};
+
   // Include the auto-generated part of this class. We split this because
   // it means we can easily edit the non-auto generated parts right here in
   // this file instead of having to edit some template or the code generator.