Merge 192655 "MutationObserver: add a check that iterating regis..."

Merge 192655 "MutationObserver: add a check that iterating regis..."

> MutationObserver: add a check that iterating registration still exists in original set
> 
> The MutationObserver registration may be unregistered from the original set
> while iterating on the cloned set.
> Add a check so that it would only call unregister() on active registrations.
> 
> BUG=453279
> TEST=manually tested from fuzzer repro case :(
> 
> Review URL: https://codereview.chromium.org/1039523003

TBR=kouhei@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193431

[haraken, 2015-04-09 08:53:25]

Committed patchset #1 (id:1) manually as r193431.

[kouhei (in TOK), 2015-04-09 15:40:10]

lgtm thanks