IndexedDB: Test updates for store/index deletion race

Follow-up to https://codereview.chromium.org/277583002 to rework test
and add new one. createIndex-after-failure.html now matches FF behavior.

BUG=370056, 368271
TBR=ericu@chromium.org

Review URL: https://codereview.chromium.org/291243003

git-svn-id: svn://svn.chromium.org/blink/trunk@174727 bbb929c8-8fbe-4397-9dbb-9b2b20218538