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

ui/gl/gl_context_cgl.cc

 // 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.
 
 #include "ui/gl/gl_context_cgl.h"
 
 #include <OpenGL/CGLRenderers.h>
 #include <OpenGL/CGLTypes.h>
 #include <vector>
 
@@ skipping 206 matching lines @@
 }
 
 GpuPreference GLContextCGL::GetGpuPreference() {
   return gpu_preference_;
 }
 
 void ScopedCGLDestroyRendererInfo::operator()(CGLRendererInfoObj x) const {
   CGLDestroyRendererInfo(x);
 }
 
+void GLContextCGL::ForceUseOfDiscreteGPU() {
+  static CGLPixelFormatObj format = NULL;
+  if (format)
+    return;
+  CGLPixelFormatAttribute attribs[1];
+  attribs[0] = static_cast<CGLPixelFormatAttribute>(0);
+  GLint num_pixel_formats = 0;
+  CGLChoosePixelFormat(attribs, &format, &num_pixel_formats);
+  // format is deliberately leaked.
+}
+
 }  // namespace gfx