Adds MapBufferSubData and MapTexSubImage2D....

gpu/command_buffer/client/cmd_buffer_helper.h

Index: gpu/command_buffer/client/cmd_buffer_helper.h
===================================================================
--- gpu/command_buffer/client/cmd_buffer_helper.h	(revision 52147)
+++ gpu/command_buffer/client/cmd_buffer_helper.h	(working copy)
@@ -109,6 +109,10 @@
     return *reinterpret_cast<T*>(data);
   }
 
+  int32 last_token_read() const {
+    return last_token_read_;
+  }
+
   error::Error GetError();
 
   // Common Commands
@@ -196,6 +200,10 @@
              shared_memory_offset);
   }
 
+  CommandBuffer* command_buffer() const {
+    return command_buffer_;
+  }
+
  private:
   // Waits until get changes, updating the value of get_.
   void WaitForGetChange();