Make Window.offscreenBuffering non-enumerable

Make Window.offscreenBuffering non-enumerable

This is in order to see how much enumeration contributes to the 0.18% usage:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/0LWNyj-edsQ/3TQq5vDyTHoJ

BUG=438370
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=186405

[philipj_slow, 2014-12-02 21:57:33]

philipj@opera.com changed reviewers:
+ chrishtr@chromium.org

[philipj_slow, 2014-12-02 21:57:34]

PTAL

[chrishtr, 2014-12-02 22:06:47]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2014-12-02 22:06:50]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2014-12-02 22:06:51]

lgtm

[commit-bot: I haz the power, 2014-12-02 22:07:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/777453003/1

[commit-bot: I haz the power, 2014-12-02 23:24:49]

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

[commit-bot: I haz the power, 2014-12-02 23:24:50]

Try jobs failed on following builders:
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/3...)

[philipj_slow, 2014-12-03 08:53:08]

update expectations

[philipj_slow, 2014-12-03 08:53:10]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2014-12-03 08:53:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/777453003/20001

[commit-bot: I haz the power, 2014-12-03 10:28:00]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=186405