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This is a sort of prerequisite for:
https://chromium-review.googlesource.com/c/chromium/src/+/1172290

The main goal is after sending:
  URLLoaderClient::OnReceiveResponse(response_header)
to ALWAYS send:
  URLLoaderClient::OnStartLoadingResponseBody(response_body)

Some URLLoader, when the response's body body is empty, don't even try to send
the response_body data pipe. In the URLLoaderClient, not having to handle the
case with no data pipe would be a nice simplification.

What this CL does:
1) In BlobUrlLoader, create the data pipe at ::Start() time.
2) Pass the producer handle to the MojoBlobCreate at construction time.
3) Pass the consumer handle to OnStartLoadingResponseBody(), immediately
   after OnReceiveResponse().

Some code simplification was possible. There is no more need to
implement PassDataPipe() in BlobURLLoader, ReaderDelegate and
DataPipeReaderDelegate. There is no more need to give them the pipe at
construction time neither.

Bug: 826868, 831155
Change-Id: I827cb39bdd782624ff362874e98be90cab0d47f3
Reviewed-on: https://chromium-review.googlesource.com/c/1329741Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607266}