Test for prefetch cache resetting on IDBObjectStore.clear()

Depends on crrev.com/116363006 on the Chromium side.

BUG=329971

Review URL: https://codereview.chromium.org/101503003

git-svn-id: svn://svn.chromium.org/blink/trunk@164246 bbb929c8-8fbe-4397-9dbb-9b2b20218538