2D Canvas: disable rendering deferral in cases with a lot of overdraw

2D Canvas: disable rendering deferral in cases with a lot of overdraw

Accumulating large draw command backlogs can result in janky
performance, and in extreme cases, a GPU denial of service. This
change fixes that problem by disabling deferred rendering when
high overdraw is detected. Cases where the overdraw overwrites
the entire canvas are not affected by this change and will
stay in deferred rendering mode because overwriting the entire
canvas resets the backlog.

BUG=522764, 522972
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201036

[Justin Novosad, 2015-08-21 18:31:20]

junov@chromium.org changed reviewers:
+ senorblanco@chromium.org

[Justin Novosad, 2015-08-21 18:31:21]

PTAL

[Stephen White, 2015-08-21 19:16:12]

LGTM

[Justin Novosad, 2015-08-21 19:23:46]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2015-08-21 19:24:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308853003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308853003/1

[commit-bot: I haz the power, 2015-08-21 22:33:33]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-21 22:33:34]

Try jobs failed on following builders:
  linux_android_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_android_r...)

[Stephen White, 2015-08-22 01:31:51]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2015-08-22 01:32:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1308853003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1308853003/1

[commit-bot: I haz the power, 2015-08-22 03:34:32]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201036