Make dispatchIfAlive ensure the life of its queue.

Make dispatchIfAlive ensure the life of its queue.

This is a speculative attempt to fix Issue 456636.

CustomElementSyncMicrotaskRunQueue::dispatchIfAlive checks that the
object is alive when it starts running callbacks, but doesn't ensure
the object lives for the duration of the callbacks. Since the
callbacks can run author JavaScript, the assumption that whatever kept
the object alive will continue to do so might be too optimistic.

Additionally, strengthen the assertion that queue processing is
non-reentrant.

BUG=456636
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190720

[dominicc (has gone to gerrit), 2015-02-17 03:40:22]

dominicc@chromium.org changed reviewers:
+ morrita@chromium.org

[dominicc (has gone to gerrit), 2015-02-17 03:41:02]

PTAL, morrita, you're probably the best (only?) person to review this. Ideas
welcome--I'll CC you on the bug now.

[Hajime Morrita, 2015-02-23 20:42:20]

LGTM.
I think kochi-san is looking into similar bug but not sure if it is the same
one.

[dominicc (has gone to gerrit), 2015-02-24 07:19:20]

The CQ bit was checked by dominicc@chromium.org

[commit-bot: I haz the power, 2015-02-24 07:20:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/916403003/1

[commit-bot: I haz the power, 2015-02-24 07:23:17]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190720