GitLab

Background:
* We are migrating callers of ConvertPathToArcUrl (sync) to using ConvertToContentUrl (async) instead.
* exo_parts.cc (a caller of ConvertToContentUrl) needs to handle multiple URLs at a time,
  so it converts single-URL callbacks to multiple-URLs callback using BarrierClosure.
* It turns out arc_file_tasks.cc (a caller of ConvertPathToArcUrl) also needs to handle multiple URLs at a time.
* Moving the BarrierClosure logic from exo_parts to path_util, so we don't need to duplicate the same BarrierClosure logic in arc_file_tasks after migration.

Bug: chromium:811679
Test: unit_test
Change-Id: I461651bac3e5110afdd7b97d39576436cb3a92fd
Reviewed-on: https://chromium-review.googlesource.com/983335Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Daichi Hirono <hirono@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547406}