[IndexedDB] Refactoring BlobWriteCallback class into a OnceCallback

The BlobWriteCallback class was created before the Callback/OnceCallback
constructs were used in Chromium. This patch converts that codepath to
use OnceCallback and lambdas to clean up the code and make it more
readable.

R=cmp@chromium.org, pwnall@chromium.org

Bug: 862456
Change-Id: I39cdb21bdb3532a191bb0c8d683c5366dbaccf92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601558
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663273}