Honor --gpu-switching=force_discrete to always stay on discrete GPU.

ui/gl/gpu_switching_manager.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_GL_GPU_SWITCHING_MANAGER_H_
 #define UI_GL_GPU_SWITCHING_MANAGER_H_
 
 #include <vector>
 
 #include "base/memory/singleton.h"
+#if defined(OS_MACOSX)
+#include "ui/gl/gl_context_cgl.h"
+#endif  // OS_MACOSX
 #include "ui/gl/gl_export.h"
 #include "ui/gl/gpu_preference.h"
 
 namespace gfx {
 
 class GL_EXPORT GpuSwitchingManager {
  public:
   // Getter for the singleton. This will return NULL on failure.
   static GpuSwitchingManager* GetInstance();
 
   // Set the switching option, but don't take any actions until later
   // we access GPU.
   void ForceUseOfIntegratedGpu();
   void ForceUseOfDiscreteGpu();
 
-  // Adjust GpuPreference based on the current switching option.
-  // If none is set, return the original GpuPreference.
+  // If no GPU is forced, return the original GpuPreference; otherwise, return
+  // the forced GPU.
+  // If forcing discrete GPU on Mac, we set up a pixel format, which keeps
+  // Chrome on the discrete GPU throughout the rest of Chrome's lifetime.
   GpuPreference AdjustGpuPreference(GpuPreference gpu_preference);
 
  private:
   friend struct DefaultSingletonTraits<GpuSwitchingManager>;
 
   GpuSwitchingManager();
   virtual ~GpuSwitchingManager();
 
   std::vector<GpuPreference> gpu_switching_option_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuSwitchingManager);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GPU_SWITCHING_MANAGER_H_