Add field trial and command line flag for Websockets over HTTP/2.

The draft is in an early stage, and a lot of manual interop testing will
have to be done before this feature can be rolled out, hence the command
line flag in addition to the field trial parameter.

Bug: 801564
Change-Id: I6fad7f0c8641f7ba4167302b121a971ddb41bd11
Reviewed-on: https://chromium-review.googlesource.com/864905
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531532}
6 files changed