[OnionSoup] Remove typemapping of (blink.mojom.Referrer, content::Referrer)

This CL removes the typemapping of
(blink.mojom.Referrer, content::Referrer) and leaves a TODO: replace
content::Referrer with blink::mojom::Referrer everywhere in the future.

Next step we can move blink/public/platform/referrer.mojom into
blink/public/mojom/.

BUG=789854

Change-Id: I70b52fe762a858d6425177604749ce4e85a7fc9c
Reviewed-on: https://chromium-review.googlesource.com/c/1317348
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Leon Han <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#605603}
19 files changed