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Includes documentation for recent changes for native editing, fast by
default, and channel support.

Bug: 620034
Change-Id: Ie6570a4c032853a4bfdfd3443091ea7f698e8e99
Reviewed-on: https://chromium-review.googlesource.com/1054333
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557653}