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In order to make crosapi more isolated from actual ash/lacros
implementations, this CL renames AshChromeService and its
corresponding variables to Crosapi.
Also, AshChromeServiceImpl is renamed to CrosapiAsh,
following the common naming convention in Chrome that
it is the crosapi implementation in Ash.

BUG=1148448
TEST=Launch lacros on a device.

Change-Id: I600188445949a4de60325f5419f137f61360d0e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640523
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845941}