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While migrating ArcService into BrowserContextKeyedService,
it turned out the boilerplate looks much bigger than what we expected.

To minimize it reasonably, this CL introduces
ArcBrowserContextKeyedServiceFactoryBase and use it for
already migrated service classes.

BUG=739097
TEST=Ran trybot. Ran on DUT.

Change-Id: I13a1e7019c06ea64fa48cec6ecf7a9e589e84b00
Reviewed-on: https://chromium-review.googlesource.com/569847Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486759}