Windows Native Notifications: Enable flag toggling on Windows.

This allows the user to manually opt into using Windows native
notifications instead of Chrome's version of the notifications.

Note: This flag is ignored on versions of Windows old enough
not to support native notifications.

Bug: 734095, 516147
Change-Id: Ic5475e6fa13e29207a5d417efc996f16fdf997db
Reviewed-on: https://chromium-review.googlesource.com/955244
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541904}
1 file changed