Add a flag to BUILDCONFIG for updating some global configs.

This is part one of a multi-way landing, as described in the comment for the added flag.

The patch that this enables is https://codereview.chromium.org/1324623005/ which will be landed after ffmpeg and WebKit have been updated.

Review URL: https://codereview.chromium.org/1312363011

Cr-Commit-Position: refs/heads/master@{#347495}
1 file changed