Proper fix for crash when checking for suspended MutationObservers

The original fix, in r183827, accidentally caused observers in "dead"
contexts to be counted as suspended. But since those contexts were gone,
we never attemped to resume the observers, thus leaking them (and their
MutationRecords) forever.

This patch inverts the executionContext() NULL check, treating observers
with NULL ExecutionContexts as "not suspended".

BUG=410755,424736
R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/665173003

git-svn-id: svn://svn.chromium.org/blink/trunk@184027 bbb929c8-8fbe-4397-9dbb-9b2b20218538