Add the Blink APIs for the WebNotificationManager.

The WebNotificationManager is analogous to the WebNotificationPresenter
living in public/web/, but without any dependency on RenderView or
RenderFrames. It also splits up the concept of the data associated with
a notification and the delegate allowing the embedder to interact with
it, as it will soon become possible for the embedder to trigger creation
of Notification objects as well (for event delivery in Service Workers).

BUG=392187

Review URL: https://codereview.chromium.org/672183003

git-svn-id: svn://svn.chromium.org/blink/trunk@184306 bbb929c8-8fbe-4397-9dbb-9b2b20218538