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After https://chromium-review.googlesource.com/c/chromium/src/+/581748
we no longer flatten SharedBuffers in GetRequestBodyForWebHTTPBody()
but instead pass segmented data downstream (multiple DataElements).

This minimizes unnecessary reallocations/copies in general, but relies
on downstream clients being able to handle segmented data (or flatten it
on the fly if really needed).

Turns out WebRequest's FormDataParsers assume the input buffers are
flattened (or segmented along expected chunk boundaries for multipart).

This CL adds a buffering mechanism to ParsedDataPresenter which flattens
consecutive byte segments before passing down to the parser.

BUG=831169

Change-Id: Ibea9193ff783cad0c80bb9722f719ff063b65720
Reviewed-on: https://chromium-review.googlesource.com/1011322
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551344}