[CrOS Messages] Add flag for web push in ChromeOS - Messages itegration

This flag controls FCM web push based background notifications for
Android Messages integration on ChromeOS. See go/awm-cros-fcm.

Bug: 920831
Change-Id: I89fc2642e60697d265fbcf0c50ea4afd6ba0de8b
Reviewed-on: https://chromium-review.googlesource.com/c/1419223
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625037}
7 files changed