Update SkPicture cull rects with RTree information

Update SkPicture cull rects with RTree information

When computed, the RTree for an SkPicture will have a root
bounds that reflects the best bounding information available,
rather than the best estimate at the time the picture recorder
is created. Given that creators frequently don't know ahead of
time what will be drawn, the RTree bound is often tighter.

Perf testing on Chrome indicates a small raster performance
advantage. For upcoming painting changes in Chrome the
performance advantage is much larger.

BUG=

Review URL: https://codereview.chromium.org/971803002