[Promise] Clear the resolver when ExecutionContext is stopped.

The resolver should be cleared when the execution context is stopped
in order to avoid leaks.
This bug was introduced at r176589.

BUG=386479

Review URL: https://codereview.chromium.org/356473004

git-svn-id: svn://svn.chromium.org/blink/trunk@176846 bbb929c8-8fbe-4397-9dbb-9b2b20218538