Implement blacklist's force GPU capability in dual GPU machines.

ui/gl/gl_context_cgl.h

Index: ui/gl/gl_context_cgl.h
===================================================================
--- ui/gl/gl_context_cgl.h	(revision 156195)
+++ ui/gl/gl_context_cgl.h	(working copy)
@@ -32,15 +32,8 @@
   virtual ~GLContextCGL();
 
  private:
-  // Expose ForceUseOfDiscreteGPU only to GLContext implementation.
-  friend class GLContext;
-
   GpuPreference GetGpuPreference();
 
-  // Helper for dual-GPU support on systems where this is necessary
-  // for stability reasons.
-  static void ForceUseOfDiscreteGPU();
-
   void* context_;
   GpuPreference gpu_preference_;