[OnionSoup] Remove the Mojo method ServiceWorkerFetchResponseCallback.OnResponseBlob

This CL is a follow-up of
https://chromium-review.googlesource.com/c/chromium/src/+/1155999, after
that we can just transfer the blob body within FetchAPIResponse.blob, so
we no longer need this OnResponseBlob method to transfer the blob body
separately, just OnResponse is enough.

BUG=789854

Change-Id: Id54f5d6ded3576bedc2e7dcf544ed41ac7715069
Reviewed-on: https://chromium-review.googlesource.com/1163288
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#581118}
13 files changed