Implement HTTP/2 support in WebSocketStream.

Implement HTTP/2 support in websocket_stream.cc::(anonymous
namespace)::Delegate::OnResponseStarted().

Convert many WebSocketStream tests to test over HTTP/2 as well.

Relax requirement for non-empty header keys in SpdyTestUtil, since
URLRequest-based tests send an empty User-Agent header.

Enable WebSockets over HTTP/2 and set some other
HttpNetworkSession::Params flags customary for tests in
WebSocketTestURLRequestContextHost constructor (copied from
SpdySessionDependencies constructor).

Bug: 801564
Change-Id: I24f5bc999c4e62dc1e8feb4b7db94d0f5b38323c
Reviewed-on: https://chromium-review.googlesource.com/940463
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540540}
4 files changed