Promote prefers-reduced-motion to stable

There is now support for prefers-reduced-motion on Android, Windows, and
Mac OSX. Linux and ChromeOS support are still pending underlying system
support on those platforms, but this should be enough to launch the
feature.

Intent to ship: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/NZ3c9d4ivA8/o4lAsCUrEQAJ

Bug: 722548
Change-Id: I3d1a7c10ed1b42cd9e007ceb6510db0a3c9f23f5
Reviewed-on: https://chromium-review.googlesource.com/c/1455742
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629678}
4 files changed