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This makes it clear that arc_vm_client_adapter.h should only have
factory functions and testing functions.

BUG=None
TEST=try

Change-Id: I47d70028d22a4c376eac3079a0523e8a4c38a59e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594791Reviewed-by: Willie Koomson <wvk@google.com>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837784}