Extend all BackgroundFetchDelegate::Client methods to take job_unique_id
as well as download_guid in order to identify to quickly identify the
job that the download belongs to. Further, BackgroundFetchDelegateProxy
now does a two stage lookup to find a download using the job id and then
the download id.

Add OnJobCancelled to BackgroundFetchDelegate::Client, which is called
in response to tapping Cancel in the Background Fetch download
notification. This method cancels the entire job rather than just an
individual download request within it. Ultimately this calls
AbortFromUser on the JobController which then signals back to
Javascript.

BackgroundFetchDelegateImpl now updates the notification after Abort()
to mark it cancelled. It is also now more resilient to calls from the
DownloadService about GUIDs that it doesn't know about. In all cases it
just drops them on the floor since it no longer has any state for them.
This mostly happens because BackgroundFetch is not yet persisting state
between browser restarts and so this will be not be a problem in the
future.

Bug: 758562
Change-Id: I8c7c70da6bf91d9b0580ec2bf8614ccc4e208da3
Reviewed-on: https://chromium-review.googlesource.com/731084


Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512300}