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This adds permission checks to make sure that the user has granted
appropriate permission for the clipboard API methods.

Bug: 677564
Change-Id: I5af221b0e18d45509ff2a8c3856680e598a51dcb
Reviewed-on: https://chromium-review.googlesource.com/804973Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525516}