Add warning message when disabling ARC.

BUG=610941
TEST=cros autotest to be submitted in
https://chrome-internal-review.googlesource.com/#/c/262336
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2002253004
Cr-Commit-Position: refs/heads/master@{#399824}