Add kClipboardContentSetting feature flag for Async Clipboard.

This will be used to control the visibility of the Clipboard Content Setting (coming in a follow-up cl).

Bug: 677564
Change-Id: I80bfcac5b5312d657046b585d82b2e58bd7e315b
Reviewed-on: https://chromium-review.googlesource.com/758459
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516177}
6 files changed