Move HPACK implementation to net/spdy/hpack directory.

This CL lands internal change 99030754 by rjshade.

BUG=488484

Review URL: https://codereview.chromium.org/1264253004

Cr-Commit-Position: refs/heads/master@{#341659}
39 files changed