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MutationObserver::DeliverMutations is a static function and
registered to the microtask queue.  It maintains active
observers and also makes active observers alive, but nothing
else (the underlying callback function nor execution context).

So, when DeliverMutations invokes each active observer,
DeliverMutations must make sure that each active observer's
execution context is still available.

This patch adds the check of the execution context.

Bug: 788495, 789862
Change-Id: Ic350603cc8a7b13275adf042e731fdd9605373aa
Reviewed-on: https://chromium-review.googlesource.com/799653
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520509}