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This adds an entry to about:flags so that users, and internal testers,
can opt into Chrome OS component updates before we roll them out to all
users.

This was originally landed in https://codereview.chromium.org/2544133002,
but reverted in https://codereview.chromium.org/2567773002
because one of the other flags did not work properly.

BUG=670421

Review-Url: https://codereview.chromium.org/2565043002
Cr-Commit-Position: refs/heads/master@{#437774}