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

ui/gl/gl_switches.h

 // 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.
 
 #ifndef UI_GL_GL_SWITCHES_H_
 #define UI_GL_GL_SWITCHES_H_
 
 // Defines all the command-line switches used by ui/gl.
 
 #include "ui/gl/gl_export.h"
 
 namespace gfx {
 
 // The GL implementation names that can be passed to --use-gl.
 GL_EXPORT extern const char kGLImplementationDesktopName[];
 GL_EXPORT extern const char kGLImplementationOSMesaName[];
 GL_EXPORT extern const char kGLImplementationAppleName[];
 GL_EXPORT extern const char kGLImplementationEGLName[];
 GL_EXPORT extern const char kGLImplementationSwiftShaderName[];
 extern const char kGLImplementationMockName[];
 
 }  // namespace gfx
 
 namespace switches {
 
-GL_EXPORT extern const char kDisableGpuSwitching[];
 GL_EXPORT extern const char kDisableGpuVsync[];
 GL_EXPORT extern const char kEnableGPUServiceLogging[];
 GL_EXPORT extern const char kEnableGPUClientLogging[];
 GL_EXPORT extern const char kGpuNoContextLost[];
 GL_EXPORT extern const char kGpuSwapDelay[];
 
 GL_EXPORT extern const char kGpuSwitching[];
 
 // The GPU switching names that can be passed to --gpu-switching.
 GL_EXPORT extern const char kGpuSwitchingOptionNameForceIntegrated[];
 GL_EXPORT extern const char kGpuSwitchingOptionNameForceDiscrete[];
 // The last one (automatic) is not used as commandline switch option.
 GL_EXPORT extern const char kGpuSwitchingOptionNameAutomatic[];
 
 GL_EXPORT extern const char kUseGL[];
 GL_EXPORT extern const char kSwiftShaderPath[];
 GL_EXPORT extern const char kTestGLLib[];
 
 }  // namespace switches
 
 #endif  // UI_GL_GL_SWITCHES_H_