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

ui/gl/gl_context_mac.mm

Index: ui/gl/gl_context_mac.mm
===================================================================
--- ui/gl/gl_context_mac.mm	(revision 158081)
+++ ui/gl/gl_context_mac.mm	(working copy)
@@ -150,11 +150,11 @@
 
     const int kMacBookProFirstDualAMDIntelGPUModel = 8;
 
+    // Do not overwrite commandline switches to honor a user's decision.
     bool forcibly_disable =
         ((model == "MacBookPro") &&
-         (model_major < kMacBookProFirstDualAMDIntelGPUModel)) ||
-        CommandLine::ForCurrentProcess()->HasSwitch(
-            switches::kDisableGpuSwitching);
+         (model_major < kMacBookProFirstDualAMDIntelGPUModel)) &&

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

It would be really nice -- though probably difficult -- to express this logic in
terms of entries in the blacklist.

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

Yes, I am thinking of supporting Mac Models in blacklist.  Will put it on my
TODO list.

+        !CommandLine::ForCurrentProcess()->HasSwitch(switches::kGpuSwitching);
 
     if (forcibly_disable) {
       GpuSwitchingManager::GetInstance()->ForceUseOfDiscreteGpu();