Enable text-only parts of Async Clipboard API by default

Previously, we RuntimeEnable'd the entire |clipboard| object on the
navigator so that the entire feature is disabled.

To launch the text-only APIs, this cl enables the |clipboard|
object, and disables all the non-text APIs.

Launch bug: crbug.com/677565

Launch review approval: https://groups.google.com/a/google.com/forum/#!topic/chrome-launch-review/ux6GtFyICVs


Bug: 677564
Change-Id: Iee7a21f71eacb4521f83afbdb23c22b37b0b5881
Reviewed-on: https://chromium-review.googlesource.com/865516
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539666}
4 files changed