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

git-svn-id: svn://svn.chromium.org/blink/trunk@192655 bbb929c8-8fbe-4397-9dbb-9b2b20218538