[OnionSoup] de-couple typemapping of content::SWFetchRequest

This CL de-couples typemapping between content::SWFetchRequest and
blink::mojom::FetchAPIRequest. The target of this work is removing
content::SWFetchRequest.

Since content::SWFetchRequest is a widely used class. Removing it in one
CL resulting in unacceptable large size of CL.

In order to seperate the work into sereral steps. A converter was added
to make convertion between two structures in this CL.
In my plan, content::SWFetchRequest will be removed in a series of CLs
gradually.

The ultimate target of the whole work is moving fetch_api_request.mojom into
third_party/blink/public/mojom/fetch/ folder. To accomplish the goal, the
whole work is seperated into six parts.

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

2. Remove content::ServiceWorkerFetchRequest. Use
blink.mojom.FetchAPIRequest instead.  --------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 that folder.


Change-Id: I2b299bd8b2830afffefe111716170c1a29e41098
Bug: 789854
Reviewed-on: https://chromium-review.googlesource.com/c/1288084
Commit-Queue: Richard Li <richard.li@intel.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603891}
26 files changed