[OnionSoup] Remove content::SWFetchRequest in background_fetch/ folder

The target of this CL is removing content::SWFetchRequest and use
blink::mojom::FetchAPIRequest instead. To keep a minor size of CL, the
operation is restricted in content/browser/background_fetch/. The final
goal is removing content::SWFetchRequest completely.

As a part of work of moving fetch_api_request.mojom, the roadmap is as
follow:

1. Remove content::RequestContextType. Use blink.mojom.RequestContextType
instead. --------- CL:1229704

2. Remove content::ServiceWorkerFetchRequest. Use
blink.mojom.FetchAPIRequest instead.  --------(CL:1288084 and This CL)

3. Remove blink::WebURLRequest::RequestContext. Use
blink.mojom.RequestContextType instead. --------- CL:1242301

4. Remove blink::WebServiceWorkerRequest. Use
blink.mojom.FetchAPIRequest instead.

5. Remove blink::WebReferrerPolicy. Use blink::mojom::ReferrerPolicy
instead.  --------- CL:1212345

6. Remove content::Referrer, blink::Referrer Use blink::mojom::Referrer
instead.

Once all the above tasks get done, fetch_api_request.mojom can be moved
into third_party/blink/public/mojom/fetch/.

Bug: 789854
Change-Id: I9b442b3329293b21d483f468c3b4264526104b85
Reviewed-on: https://chromium-review.googlesource.com/c/1312673
Commit-Queue: Richard Li <richard.li@intel.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606707}
36 files changed