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

ui/gl/gpu_switching_manager.h

Index: ui/gl/gpu_switching_manager.h
===================================================================
--- ui/gl/gpu_switching_manager.h	(revision 158081)
+++ ui/gl/gpu_switching_manager.h	(working copy)
@@ -8,6 +8,9 @@
 #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"
 
@@ -23,8 +26,13 @@
   void ForceUseOfIntegratedGpu();
   void ForceUseOfDiscreteGpu();
 
-  // Adjust GpuPreference based on the current switching option.
-  // If none is set, return the original GpuPreference.
+  // On Mac:
+  //   If no GPU is forced, return the original GpuPreference.
+  //   Otherwise, always return PreferIntegratedGpu.

[Ken Russell (switch to Gerrit), 2012/09/21 23:00:29]

I understand why you're returning PreferIntegratedGpu all the time in this case
-- to avoid redundant allocations of pixel formats forcing the system onto the
discrete GPU -- but I think that behavior will be confusing when debugging
things later. I think you should return gpu_switching_option_[0] despite any
inefficiency.

[Zhenyao Mo, 2012/09/21 23:08:09]

Done.

+  //   If forcing discrete GPU, we set up a pixel format, which keeps
+  //   Chrome on the discrete GPU throughout the rest of Chrome's lifetime.
+  // On other platforms:
+  //   Return the original GpuPreference.
   GpuPreference AdjustGpuPreference(GpuPreference gpu_preference);
 
  private: