UI impl side painting default on

Turn on Impl Side Painting on Aura browser compositor by default
and add a disable compositor switch.

(Last attempt to re-land is at https://codereview.chromium.org/1160093005/, the try bot failure seem to be fixed with https://codereview.chromium.org/1166983007/ which is committed)

R=danakj
BUG=314185

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/962833003

Cr-Commit-Position: refs/heads/master@{#333656}
3 files changed