bindings: Makes DOMArrayBufferDeallocationObserver::instance thread-safe.

Issue 428437 seems caused by a race to initialize the global instance
and to return it among multiple threads.
Makes the initialization thread-safe.

BUG=428437

Review URL: https://codereview.chromium.org/690893003

git-svn-id: svn://svn.chromium.org/blink/trunk@184640 bbb929c8-8fbe-4397-9dbb-9b2b20218538