Use promises in notifications tests and enable controlling the page.
For Web Notifications layout tests:
* sendCommand returns a promise, sends one command, and waits for a response.
* simulateNotificationClick returns a promise, simulates a click, and waits for a response.
* Make it possible to control the test page when using ./ scope.
Committed:
https://crrev.com/9725447a246150f32bf88cfe71eee37ccb91abeb
Cr-Commit-Position: refs/heads/master@{#389069}