Remove WebIDBFactoryImpl IOThreadHelper class

Now that the renderer-side factory uses a normal Mojo interface,
there's no longer a need to jump to the IO thread to make the
calls on that interface.  This allows us to clean up the code
by removing the IO thread helper class.

Bug: 717812
Change-Id: Iab04a90677d50ab3a911330ede2333d452b6f829
Reviewed-on: https://chromium-review.googlesource.com/1150724
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578939}
2 files changed