Don't deref XMLHttpRequest synchronously below stop()

Don't deref XMLHttpRequest synchronously below stop()

Prior to this CL, we could release our last reference to XMLHttpRequest
synchronously below XMLHttpRequest::stop. That can cause us to call
ScriptExecutionContext::willDestroyActiveDOMObject synchronously below stop(),
which will hit a CRASH() because we're trying to mutate the set of
ActiveDOMObject while we're iterating the set.

This CL simply makes the deref() happen asynchronously, preventing us from
mutating the set during iteration.

BUG=247395
R=eseidel

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=152266

[eseidel, 2013-06-12 00:11:39]

lgtm

I believe you this is an OK fix.  I wish we had a test or understood how to
test.

[abarth-chromium, 2013-06-12 00:45:21]

On 2013/06/12 00:11:39, eseidel wrote:
> I wish we had a test or understood how to test.

I'm sorry I had to run from the office.  I'm going to put this into the
commit-queue, but I'll think more about how we might test this change tonight.

[commit-bot: I haz the power, 2013-06-12 00:45:31]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/abarth@chromium.org/16284009/1

[commit-bot: I haz the power, 2013-06-12 06:39:01]

Change committed as 152266