Track the cost of impl-side painting.

cc/picture_pile_impl.h

Index: cc/picture_pile_impl.h
===================================================================
--- cc/picture_pile_impl.h	(revision 170195)
+++ cc/picture_pile_impl.h	(working copy)
@@ -27,6 +27,8 @@
   // It is assumed that contentsScale has already been applied to this canvas.
   void Raster(SkCanvas* canvas, gfx::Rect rect);
 
+  double rasterizeTime();
+
 private:
   friend class PicturePile;
 
@@ -34,6 +36,7 @@
   ~PicturePileImpl();
 
   std::vector<scoped_refptr<Picture> > pile_;
+  double rasterize_time_in_seconds_;
 
   friend class base::RefCounted<PicturePileImpl>;
   DISALLOW_COPY_AND_ASSIGN(PicturePileImpl);