Mark SwapBuffers for an offscreen surface as unimplemented

gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h

Index: gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h
index 6f3fb5ca11da02dd6d35488ebba82f91ad9b481c..0193b4aa35b665c8ab5f2415686367ec3f2ea813 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough.h
@@ -316,6 +316,7 @@ class GLES2DecoderPassthroughImpl : public GLES2Decoder {
   // The GL context this decoder renders to on behalf of the client.
   scoped_refptr<gl::GLSurface> surface_;
   scoped_refptr<gl::GLContext> context_;
+  bool offscreen_;
 
   // Managers
   std::unique_ptr<ImageManager> image_manager_;