This CL adds the following metrics:

* If the system has a dbus object at /org/Freedesktop/Notifications
  with interface org.Freedesktop.Notifications
* Server capabilities as returned by GetCapabilities
  * Capabilities are stored in |capabilities_|.  It is not used now,
    but will be in the future.
* Whether native notifications are actually used

BUG=676220
R=peter@chromium.org,mpearson@chromium.org
CC=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2856753002
Cr-Commit-Position: refs/heads/master@{#469434}