Instead of repeating it in every configuration, just add it to the
common FLAGS.

R=machenbach@chromium.org

Change-Id: I93e7ef0f0ad55bfe0a0e24f50d5a73d4658d7554
Reviewed-on: https://chromium-review.googlesource.com/1141733
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54552}