Fix race between updatedocument and sendTokensToMainThread

Fix race between updatedocument and sendTokensToMainThread

Fixes a race that would occur if the blink scheduler wanted to
prioritize loading tasks.

BUG=510398
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201308

[alex clarke (OOO till 29th), 2015-08-26 17:27:01]

alexclarke@chromium.org changed reviewers:
+ kouhei@chromium.org

[alex clarke (OOO till 29th), 2015-08-26 17:27:02]

This was fun to track down. A bunch of browser tests would fail strangely if I
try to prioritize loading tasks over default tasks.  This patch fixes the race.

[Sami, 2015-08-26 17:36:03]

skyostil@chromium.org changed reviewers:
+ skyostil@chromium.org

[Sami, 2015-08-26 17:36:04]

Good find! Non-owner lgtm.

[kouhei (in TOK), 2015-08-27 04:42:59]

kouhei@chromium.org changed reviewers:
+ hiroshige@chromium.org

[kouhei (in TOK), 2015-08-27 04:43:00]

lgtm

+hiroshige: FYI

[alex clarke (OOO till 29th), 2015-08-27 09:03:36]

The CQ bit was checked by alexclarke@chromium.org

[commit-bot: I haz the power, 2015-08-27 09:03:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1318463004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1318463004/1

[commit-bot: I haz the power, 2015-08-27 11:23:18]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201308