[Mojo] Fix race condition in sample_app's SwapBuffers

mojo/examples/sample_app/sample_gles2_delegate.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/examples/sample_app/sample_gles2_delegate.h"
 
 #include <stdio.h>
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>
 
 namespace mojo {
 namespace examples {
 
 SampleGLES2Delegate::SampleGLES2Delegate()
     : gl_(NULL) {
 }
 
 SampleGLES2Delegate::~SampleGLES2Delegate() {
 }
 
 void SampleGLES2Delegate::DidCreateContext(
-    GLES2* gl, uint32_t width, uint32_t height) {
+    GLES2ClientImpl* gl, uint32_t width, uint32_t height) {
   gl_ = gl;
   cube_.Init(width, height);
   last_time_ = base::Time::Now();
   timer_.Start(FROM_HERE, base::TimeDelta::FromMilliseconds(16),
                this, &SampleGLES2Delegate::Draw);
 }
 
 void SampleGLES2Delegate::Draw() {
   base::Time now = base::Time::Now();
   base::TimeDelta offset = now - last_time_;
   last_time_ = now;
   cube_.Update(offset.InSecondsF());
   cube_.Draw();
   gl_->SwapBuffers();
 }
 
-void SampleGLES2Delegate::ContextLost(GLES2* gl) {
+void SampleGLES2Delegate::ContextLost(GLES2ClientImpl* gl) {
+  gl_ = NULL;
   timer_.Stop();
-  gl_ = NULL;
 }
 
 }  // namespace examples
 }  // namespace mojo