[OnionSoup] Move background_fetch.mojom to blink/public/mojom/

This moves background_fetch.mojom from:
blink/public/platform/modules/background_fetch/
to:
blink/public/mojom/background_fetch/

TBRing reviewers:
jochen: Please reviewing changes in chrome/browser/DEPS

TBR=jochen@chromium.org

Bug: 789854
Change-Id: I0c67ae8e944d4e2c4c91da2511f933e6e91b9f70
Reviewed-on: https://chromium-review.googlesource.com/c/1370898
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Leon Han <leon.han@intel.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Richard Li <richard.li@intel.com>
Cr-Commit-Position: refs/heads/master@{#616241}
41 files changed