Implement blacklist's force GPU capability in dual GPU machines.

content/browser/gpu/gpu_util.h

Index: content/browser/gpu/gpu_util.h
===================================================================
--- content/browser/gpu/gpu_util.h	(revision 156326)
+++ content/browser/gpu/gpu_util.h	(working copy)
@@ -11,6 +11,7 @@
 #include "build/build_config.h"
 #include "content/common/content_export.h"
 #include "content/public/common/gpu_feature_type.h"
+#include "content/public/common/gpu_switching_option.h"
 
 class GpuBlacklist;
 
@@ -32,6 +33,10 @@
 CONTENT_EXPORT content::GpuSwitchingOption StringToGpuSwitchingOption(
     const std::string& switching_string);
 
+// Gets a string version of a GpuSwitchingOption.
+CONTENT_EXPORT std::string GpuSwitchingOptionToString(
+    content::GpuSwitchingOption option);
+
 // Send UMA histograms about the enabled features.
 CONTENT_EXPORT void UpdateStats(
     const GpuBlacklist* blacklist, uint32 blacklisted_features);