cc: Replace PlaybackRanges with explicit indices.

This patch overall removes the concept of ranges and adds explicit
indices for each of the item recorded in the paint op buffer.
- Items that used to be grouped now result in repeated visual rects
  (same for each item in the group)
- PaintOpBuffer::Iterator now understands indices

R=enne@chromium.org
TBR=sadrul@chromium.org

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iec409ab69feeb7bb6fb97e43836b87fd512b4700
Reviewed-on: https://chromium-review.googlesource.com/530093
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Chris harrelson <chrishtr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480140}