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The BufferedBody CL got reverted at https://crrev.com/c/2532869 because
it introduced flakes. This CL relands it and adds a potential fix:
write the drained body gradually, as previously we won't write to the
newly created pipe until we've finished draining completely.

This CL also changes the buffered_body_ data type to use queue of chunks
so that we can free memory as soon as we finished writing a chunk of
the response body to prevent potential OOMs, and protects all of this
code behind a flag (later on we'll also add a check so that we'll buffer
the body only when we're deferring due to bfcache)

Bug: 1147081, 1148130, 1137682
Change-Id: I61d95c9c67220a296daa66f7409f27fdeb0e8182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531338Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827172}