Remove WebIDB{Database,Cursor}Impl IOThreadHelper class

This is the next step in removing IO thread hopping from the
renderer-side non-Blink IDB code.

Bug: 717812
Change-Id: I4f0bfc8d98ad7bd9d041954f74215f75271702f9
Reviewed-on: https://chromium-review.googlesource.com/1155794
Reviewed-by: Chris Mumford <cmumford@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580282}
5 files changed