Include *.mojom-forward.h instead of .mojom.h from fetch header files

To reduce pre-processed size of fetch_api_request_body_mojom_traits.h
and fetch_api_request_headers_mojom_traits.h, this CL replaces
.mojom.h with .mojom-forward.h.

This CL has no behavior changes.

Bug: 1001360
Change-Id: I1360771e268e7e079b9d9f05685c7af138c4f0e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1806533
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#697867}