Let Notification Handlers report when event handling has completed

This will significantly help reduce flakiness in testing (one deflake
included in this CL), and is required for migrating Android's intent
handling system to the BackgroundTaskScheduler which needs to know when
handling of the event has finished.

In addition, a hack where the NativeNotificationDisplayService invoked
the Close() method on a notification's delegate in order to fire the
event for non-persistent Web Notifications has been removed. We can just
fire the event from the NotificationMessageFilter instead.

BUG=766854, 784951

Change-Id: I57d0a446e6c47281bb7b5e001c449fadddeba571
Reviewed-on: https://chromium-review.googlesource.com/779722


Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518305}