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On the way to creating a BackgroundFetchDelegate, factor out the
thread-crossing and Core part of BackgroundFetchJobController into
BackgroundFetchDelegateProxy. Ultimately this will have very little business
logic but that will come in a follow-up.

TBR=avi@chromium.org

Change-Id: Iaa4cc9491a1758b8cd3c64a31cc41407c701ebaa
Reviewed-on: https://chromium-review.googlesource.com/571801
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: John Mellor <johnme@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487464}