[Background Fetch] Create a mojo interface for registration-tied methods

Previously all mojo calls went through a global interface. This change
makes the `Fetch` and `Get` calls return a registration with a
registration service interface, if other registration related mojo calls
need to be made (like Abort).

The unique_id was also removed from the mojo struct / renderer since
it is no longer needed to identify registrations.

The WebBackgroundFetchRegistration which is used to dispatch events was
also updated to include the registration service interface.

Bug: 866874
Change-Id: I9feab9f6c5238ec39d61c55aa5a43b9122c0879d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533880
Commit-Queue: Rayan Kanso <[email protected]>
Reviewed-by: Peter Beverloo <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Mugdha Lakhani <[email protected]>
Cr-Commit-Position: refs/heads/master@{#647695}
51 files changed