[KeyboardMap] Enable navigator.keyboard.getLayoutMap() on Stable.

Launch bug (w/ approvals): crbug.com/831743

Intent 2 ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V2xR7cucnOA

Bug: 832811, 831743
Change-Id: I24d2f49e82b5afe1dd668dc57e0ceb98af36e205
Reviewed-on: https://chromium-review.googlesource.com/1141104
Reviewed-by: Alex Russell <slightlyoff@chromium.org>
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576796}
3 files changed