Add a command-line flag to enable threaded GPU rasterization.

content/renderer/gpu/compositor_dependencies.h

Index: content/renderer/gpu/compositor_dependencies.h
diff --git a/content/renderer/gpu/compositor_dependencies.h b/content/renderer/gpu/compositor_dependencies.h
index 26dff9d83c8c42d68aedab99152cc92a328419b8..9aae19f2db1182fa87920adc796fdc7f1eece36e 100644
--- a/content/renderer/gpu/compositor_dependencies.h
+++ b/content/renderer/gpu/compositor_dependencies.h
@@ -30,6 +30,7 @@ class CompositorDependencies {
   virtual bool IsImplSidePaintingEnabled() = 0;
   virtual bool IsGpuRasterizationForced() = 0;
   virtual bool IsGpuRasterizationEnabled() = 0;
+  virtual bool IsThreadedGpuRasterizationEnabled() = 0;
   virtual int GetGpuRasterizationMSAASampleCount() = 0;
   virtual bool IsLcdTextEnabled() = 0;
   virtual bool IsDistanceFieldTextEnabled() = 0;