Media Galleries: Port GalleryWatchManager off of FILE thread.

Move to using a SequencedTaskRunner instead, as the calls still need
to be sequenced with each other (not run concurrently).

Bug: 689520
Change-Id: I59dc2bffd753da8c6e68598271c6070c79fab13d
Reviewed-on: https://chromium-review.googlesource.com/583575
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489674}
2 files changed