Fetch notification action icons and pass them through in resources.

* Adds an action_icons field to NotificationResources
* Turns PendingNotification into a real class.
* PendingNotification fetches potentially multiple resources before running its callback.
* Adds a unit test for this area of code.

BUG=581336,423039

Review URL: https://codereview.chromium.org/1644083002

Cr-Commit-Position: refs/heads/master@{#374881}
17 files changed