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

ui/gl/gpu_switching_manager.cc

Index: ui/gl/gpu_switching_manager.cc
===================================================================
--- ui/gl/gpu_switching_manager.cc	(revision 158081)
+++ ui/gl/gpu_switching_manager.cc	(working copy)
@@ -31,10 +31,17 @@
 
 GpuPreference GpuSwitchingManager::AdjustGpuPreference(
     GpuPreference gpu_preference) {
+#if defined(OS_MACOSX)
   if (gpu_switching_option_.size() == 0)
     return gpu_preference;
   DCHECK_EQ(gpu_switching_option_.size(), 1u);
-  return gpu_switching_option_[0];
+  if (gpu_switching_option_[0] == PreferDiscreteGpu)
+    GLContextCGL::ForceUseOfDiscreteGPU();
+  return PreferIntegratedGpu;
+#else
+  return gpu_preference;
+#endif  // OS_MACOSX
 }
 
 }  // namespace gfx
+