Linux native notifications: Support icons

This CL:
* Adds support for notification icons
* Adds an urgency hint
* Supports updating NotificationCommon::Type

BUG=676220

R=peter@chromium.org,thestig@chromium.org

Review-Url: https://codereview.chromium.org/2806203003
Cr-Commit-Position: refs/heads/master@{#464650}
2 files changed