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

This CL removes the typemapping of
(blink.mojom.Referrer, blink::Referrer), a follow-up CL will remove the
typemapping of (blink.mojom.Referrer, content::Referrer), after that we
can move blink/public/platform/referrer.mojom into blink/public/mojom/.

BUG=789854

Change-Id: I9acb000e5f64939cd5898954e85f4288e2a88678
Reviewed-on: https://chromium-review.googlesource.com/c/1317344
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Leon Han <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#605265}
5 files changed