Speculative crash fix on IDBRequest.

There are possibility that IDBRequest::m_requestState can already be
cleared on IDBRequest::dispatchEvent() when the execution context
is gone and IDBRequest::stop() is notified before the dispatchEvent().

No test. This is a fix based on reported crash data.

BUG=325618
TEST=none
R=haraken

Review URL: https://codereview.chromium.org/106103004

git-svn-id: svn://svn.chromium.org/blink/trunk@163236 bbb929c8-8fbe-4397-9dbb-9b2b20218538