Plumb Notification action icons through to the UI layer.
* This makes the feature functional on both Desktop and Android.
* It is still behind the NotificationActionIcons runtime flag.
* Stops using NotificationCompat to enable using the Icon class.
BUG=
581336
Committed:
https://crrev.com/647546723325fa08774caf23c1aa71dc15b279c1
Cr-Commit-Position: refs/heads/master@{#374918}