Creating blockfile impl of OpenOrCreate

Adding OpenOrCreate implementation to Blockfile backend.
Adding blockfile OpenOrCreate test

Change-Id: I707f4133704e9cd93c30ed0cfa6fbf670193d0ce
Reviewed-on: https://chromium-review.googlesource.com/c/1403183
Commit-Queue: Steven Bingler <bingler@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621323}
5 files changed