[OnionSoup] Move content::SWFetchRequest::Serialize to SWUtils

In this CL, function in content::SWFetchRequest::Serialize is moved into
content::ServiceWorkerUtils::SerializeRequestData.

This work is a preparation for removing content::SWFetchRequest.

BUG=789854

Change-Id: Ic0867e4ee75a19488b92a62a44cd2bad14839a63
Reviewed-on: https://chromium-review.googlesource.com/c/1270519
Commit-Queue: Richard Li <richard.li@intel.com>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599488}
11 files changed