[OnionSoup] Use FetchAPIRequest.body instead of FetchAPIRequest.blob in SW code

Previously, in non-S13nServiceWorker case, a
network::ResourceRequestBody was being converted to a blob as
FetchAPIRequest.blob to transfer data for FetchEvent#request#body
dispatched to service workers.

With crrev.com/c/1367365 introducing a new field FetchAPIRequest.body of
type network::mojom::URLRequestBody which has a typemap to
network::ResourceRequestBody, now we can use it instead to avoid the
data conversion above.

After this CL, FetchAPIRequest.blob has only one user left: Background
Fetch API implementation.

BUG=911930

Change-Id: I034e3cc539e19f5cf7b9d35809f12a54a110ac8f
Reviewed-on: https://chromium-review.googlesource.com/c/1390323
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Leon Han <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#619010}
8 files changed