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..62fcba1ba4657c3331b7c6d3cceb5e10376d8e13 100644
--- a/content/renderer/gpu/compositor_dependencies.h
+++ b/content/renderer/gpu/compositor_dependencies.h
@@ -38,6 +38,7 @@ class CompositorDependencies {
   virtual bool IsElasticOverscrollEnabled() = 0;
   // Only valid in single threaded mode.
   virtual bool UseSingleThreadScheduler() = 0;
+  virtual bool IsThreadedGpuRasterizationEnabled() = 0;

[danakj, 2015/02/02 21:48:38]

maybe move this up by other GpuRaster functions?

[vmiura, 2015/02/02 22:03:22]

Done.

   virtual uint32 GetImageTextureTarget() = 0;
   virtual scoped_refptr<base::SingleThreadTaskRunner>
   GetCompositorMainThreadTaskRunner() = 0;