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When a service worker executes code like `evt.respondWith(fetch(r))` we
should be able to directly pass the fetch's resulting mojo::DataPipe
through without any data copying.  This previously did not work,
however, since the BufferingBytesConsumer would immediately start
buffering the Response body data.

This CL fixes this issue by delaying the start of the buffering by a
short amount of time.  This gives the service worker time to drain the
pipe.

The delay is currently disabled by default behind the
"BufferingBytesConsumerDelay" feature.

Based on yhirano's draft CL at crrev.com/c/1383755.

Bug: 911036
Change-Id: I65675ce62a7ce593c8994b3e1258634840ba6c2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679669
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676053}