Fix crash in MutationObserver::canDeliver due to NULL ExecutionContext

Fix crash in MutationObserver::canDeliver due to NULL ExecutionContext

Rather than NULL-checking it directly, I've instead plumbed through
ActiveDOMCallback::canInvokeCallback(), which handles all the logic
MutationObserver was doing previously and also handles the NULL check.

The test case is crazy-looking because it was generated by ClusterFuzz.
I've simplified it as much as possible.

BUG=410755
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183827

[adamk, 2014-10-15 23:34:39]

adamk@chromium.org changed reviewers:
+ rafaelw@chromium.org

[haraken, 2014-10-16 01:00:52]

LGTM

[adamk, 2014-10-16 15:47:56]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2014-10-16 15:48:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657983002/1

[commit-bot: I haz the power, 2014-10-16 15:51:24]

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

[commit-bot: I haz the power, 2014-10-16 15:51:29]

Try jobs failed on following builders:
  mac_gpu_retina_triggered_tests on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/mac_gpu_retina_tr...)

[adamk, 2014-10-16 15:53:40]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2014-10-16 15:54:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657983002/1

[commit-bot: I haz the power, 2014-10-16 15:57:50]

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

[commit-bot: I haz the power, 2014-10-16 15:57:51]

Try jobs failed on following builders:
  mac_gpu_retina_triggered_tests on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/mac_gpu_retina_tr...)

[adamk, 2014-10-16 16:26:34]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2014-10-16 16:26:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657983002/1

[commit-bot: I haz the power, 2014-10-16 16:27:39]

Committed patchset #1 (id:1) as 183827