Linux native notifications: Add attribution

This CL adds the origin url to the notification body (if one exists),
similar to how Chrome notifications do currently.

This is one of the requirements specified in [1] (Google only).

[1] https://docs.google.com/a/google.com/document/d/1cqXPpIyCf1RaEYx9XCPjZbO6U5oDeyslajznnG9wd_U/edit?usp=sharing

BUG=676220
R=peter@chromium.org,thestig@chromium.org
TBR=mukai@chromium.org

Review-Url: https://codereview.chromium.org/2876603004
Cr-Commit-Position: refs/heads/master@{#472160}
3 files changed