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

ui/gl/gl_context_mac.mm

 // 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 "base/basictypes.h"
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 132 matching lines @@
     // system stability issues. http://crbug.com/113703
     std::string model;
     int32 model_major, model_minor;
     if (!base::mac::ParseModelIdentifier(base::mac::GetModelIdentifier(),
                                          &model, &model_major, &model_minor)) {
       return false;
     }
 
     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)) &&
+        !CommandLine::ForCurrentProcess()->HasSwitch(switches::kGpuSwitching);
 
     if (forcibly_disable) {
       GpuSwitchingManager::GetInstance()->ForceUseOfDiscreteGpu();
       return false;
     }
 
     supports_dual_gpus = true;
   }
 
   return supports_dual_gpus;
 }
 
 }  // namespace gfx