Add new HTTP/2 and HPACK decoder in net/http2/.

A follow-up CL will add flags and new classes to net/spdy
that will benefit from this new implementation.

This CL lands all server changes in the http2 directory
up to 129999723 by jamessynge.

BUG=488484

Review-Url: https://codereview.chromium.org/2293613002
Cr-Commit-Position: refs/heads/master@{#436466}
126 files changed