cc: Call PrepareTiles when becoming visible in browser only.
Calling PrepareTiles on each visibility change might have caused a v8
idle time benchmark regression. We only need to call PrepareTiles for
the SingleThreadProxy wait for ready to draw mode.
R=brianderson@chromium.org,danakj@chromium.org
TEST=LayerTreeHostTestReadyToDrawVisibility
BUG=499040
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review URL: https://codereview.chromium.org/
1239843002
Cr-Commit-Position: refs/heads/master@{#339201}