Adds a feature flag for foreground notification manager.

This CL adds a feature flag for the new foreground notification manager
system.

Bug: 916659
Change-Id: I9e0fc128a9b094ec438e3c0efcd9da50baa9e15a
Reviewed-on: https://chromium-review.googlesource.com/c/1389440
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624480}
7 files changed