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This CL migrates ArcDownloadsWatcherService.

BUG=672829
TEST=Ran try.

Change-Id: I1004dfa57616778463f36e86c3cc46295d8c9627
Reviewed-on: https://chromium-review.googlesource.com/571567Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487087}