IndexedDB Tests: Ensure worker ref is retained to avoid GC

As seen in crbug.com/338866 the Worker object can be GC'd leading to
test timeouts. Ensure that a global reference to workers is retained,
which will hopefully address the timeout flakiness in out worker tests.

R=dgrogan@chromium.org
BUG=127244,338866

Review URL: https://codereview.chromium.org/149403004

git-svn-id: svn://svn.chromium.org/blink/trunk@166860 bbb929c8-8fbe-4397-9dbb-9b2b20218538