Refactor NotificationPlatformBridgeLinux

This CL:
* Removes the dependency on gdbus from NPBL and uses //src/dbus instead
* Handle notifications asynchronously on a dedicated task runner
* Modify NativeNotificationDisplayService to allow async initialization
  of the NotificationPlatformBridge (only on Linux)

BUG=676220
R=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2821533003
Cr-Commit-Position: refs/heads/master@{#467563}
14 files changed