Add :protocol header for WebSockets over HTTP/2.

This pseudo-header is defined in
https://tools.ietf.org/html/draft-ietf-httpbis-h2-websockets-00.

This CL lands server change 184848446 by bnc.

BUG=488484

Change-Id: I32000864d0f5cfef66767b8b23ddf4749c61cfc3
Reviewed-on: https://chromium-review.googlesource.com/925624
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537848}

net/spdy/core/spdy_protocol.cc

@@ -248,6 +248,7 @@ const char* const kHttp2AuthorityHeader = ":authority";
 const char* const kHttp2MethodHeader = ":method";
 const char* const kHttp2PathHeader = ":path";
 const char* const kHttp2SchemeHeader = ":scheme";
+const char* const kHttp2ProtocolHeader = ":protocol";
 
 const char* const kHttp2StatusHeader = ":status";
 

net/spdy/core/spdy_protocol.h

@@ -322,6 +322,7 @@ SPDY_EXPORT_PRIVATE extern const char* const kHttp2AuthorityHeader;
 SPDY_EXPORT_PRIVATE extern const char* const kHttp2MethodHeader;
 SPDY_EXPORT_PRIVATE extern const char* const kHttp2PathHeader;
 SPDY_EXPORT_PRIVATE extern const char* const kHttp2SchemeHeader;
+SPDY_EXPORT_PRIVATE extern const char* const kHttp2ProtocolHeader;
 
 // Name of pseudo-header defined for HTTP/2 responses.
 SPDY_EXPORT_PRIVATE extern const char* const kHttp2StatusHeader;