Spanify net::IOBuffer, part 2

Make HttpConnection's two IOBuffer subclasses use SetSpan() and
ClearSpan() methods to modify the data tracked by the underlying
IOBuffer. This CL does not attempt to spanify those two classes' own
APIs, for the sake of simplicity.

Bug: 396621713
Change-Id: Iafb8f043e64accdb3b92b3fdb5649f2abfa636c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6270756
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422267}