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.

Review URL: https://codereview.chromium.org/1907443007

Cr-Commit-Position: refs/heads/master@{#389069}
30 files changed