Move BackgroundFetchDelegate to content/public

The implementation of this interface will ultimately come from chrome so
move it into content/public/browser as well as BackgroundFetchResponse
which it uses in the interface.

Bug: 758562
Change-Id: I214f7b22e408ecfb744aa9868b06f99ff16df2ea
Reviewed-on: https://chromium-review.googlesource.com/647751
Reviewed-by: Tobias Sargeant <[email protected]>
Reviewed-by: John Abd-El-Malek <[email protected]>
Reviewed-by: Eric Seckler <[email protected]>
Reviewed-by: John Mellor <[email protected]>
Commit-Queue: Dan Elphick <[email protected]>
Cr-Commit-Position: refs/heads/master@{#503707}
50 files changed