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This will prevent Chrome from adding 'import' lines in the future
which could cause a infinite import loop (by importing the combined
property file itself.) See crbug.com/1114279.

This makes our unit tests simpler too.

BUG=None
TEST=arc.Boot.vm, try

Change-Id: I66f58cf4fdce5e2e84de754755e9d90525eb666e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343505Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796583}