Add histogram to track server HTTP/2 WebSocket support.

Add Net.SpdySession.ServerSupportsWebSocket histogram to track server
support of WebSockets over HTTP/2.

Bug: 801564
Change-Id: I1cd63e5bdca17106487c48ce6c1734e4d6591985
Reviewed-on: https://chromium-review.googlesource.com/1152486
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579500}
3 files changed