Make Blob reading async in FakeBlobRegistry::Register()

Since mojom::BlobRegistry::Register() is declared [Sync],
doing so much in the callee could cause deadlock and
I actually see it when I tried to add tests to
form_data_bytes_consumer_test.cc using FakeBlobRegistry.
This CL makes the function thin and move almost things async.

This is a preparation to fix the bug that
uploaded FormData breaks in Service Worker.

Bug: 40790626
Change-Id: If047b7fd42818ace56368416aa7a48fd534755cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6229067
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1415389}