Add initial support for native Linux desktop notifications

This CL adds a stub implementation of NotificationPlatformBridgeLinux,
which is responsible for communicating notification changes to the desktop
environment via D-Bus.  Once this class is fully implemented, it is
intended to be used by default when the host supports notifications.

BUG=676220
R=thestig@chromium.org,yoshiki@chromium.org

Review-Url: https://codereview.chromium.org/2794103002
Cr-Commit-Position: refs/heads/master@{#461990}
11 files changed