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Instead of creating the response's body data pipe during the first call to
OnWillRead(), it is now done in OnWillStart(). The data pipe
consumer handle is available in OnResponseStarted().

It allows the MojoAsyncResourceHandler to call:
 * URLLoader::OnReceiveResponse(response_headers)
 * URLLoader::OnStartLoadingResponseBody(response_body)
at the same time.

The goal is to guarantee the response's body is always sent, even if it doesn't
contains any data.

Bug: 831155, 826868
Change-Id: I85d84d05ad3fd362d96394834e14297d550f5f13
Reviewed-on: https://chromium-review.googlesource.com/c/1352254Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612137}