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

ui/gl/gl_switches.cc

Index: ui/gl/gl_switches.cc
===================================================================
--- ui/gl/gl_switches.cc	(revision 158081)
+++ ui/gl/gl_switches.cc	(working copy)
@@ -17,11 +17,6 @@
 
 namespace switches {
 
-// Disable dynamic switching between integrated and discrete GPU on
-// systems that would otherwise support it (currently, only a limited
-// number of MacBook Pros).
-const char kDisableGpuSwitching[]           = "disable-gpu-switching";
-
 // Stop the GPU from synchronizing on the vsync before presenting.
 const char kDisableGpuVsync[]               = "disable-gpu-vsync";