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When a Background Fetch registration is created, a BGFetchJobContoller
is created immediately after committing it to storage even if the job
can't be started just yet -- this should be the scheduler's decision.

This CL is the first step in removing the assumption that each existing
Background Fetch registration is represented by a BGFetchJobController.
Acknowledging creation of the registration is separated from indicating
that a new registration is available, by adding an OnRegistrationCreated
method to the BackgroundFetchDataManagerObserver.

As part of this refactoring, in an effort to avoid passing around unique
pointers to BackgroundFetchRegistration objects, these are now passed by
const& where the receiver makes a copy when necessary. This enabled
reducing knowledge of the protobufs throughout our system, which is a
nice clean-up on its own.

TBR=avi for BUILD.gn

Bug: 850512
Change-Id: I8d3c42a11ab89f7b7c3573a6f9a21e096493b838
Reviewed-on: https://chromium-review.googlesource.com/1149871
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577903}