service worker: Eliminate DispatchLegacyFetchEvent.

This reduces the difference between non-S13nServiceWorker and S13nServiceWorker
code.

The main change is moving non-S13nSW to use ResourceRequest instead
of ServiceWorkerFetchRequest. The wrinkle is that SWFetchRequest had three
fields for the requeest body which seems like we still must use. Non-S13nSW
makes a blob out of the request body and sends it to the renderer as
blob UUID, blob size, and a Mojo blob ptr. I've retained this behavior by
passing the blob stuff in addition to ResourceRequest.

R=kinuko, shimazu

Bug: 715640
Change-Id: I633c9b414e53e8acd1f122a69d6ccb4d2d3c2f62
Reviewed-on: https://chromium-review.googlesource.com/925982
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538015}
14 files changed