Accept HEAD method for HTTP/2 push.

Currently, SpdyHttpStream::Initialize() requires that method is GET for
a request to be matched to a pushed stream, but
SpdySession::TryCreatePushedStream() allows both GET and HEAD methods.

This change removes the requirement from SpdyHttpStream::Initialize(),
and changes SpdySession::ValidatePushedStream() to require that request
method matches that of the pushed stream in ValidatePushedStream().

In order to do this, HttpRequestInfo is passed through
ClaimPushedStream() to ValidatePushedStream().  This will be useful in
subsequent CLs when the pushed request and response headers will be
validated against the request headers.

Bug: 554220
Change-Id: I37c502881a782b05e01c012dbb98352eb48fbed7
Reviewed-on: https://chromium-review.googlesource.com/860480
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528622}
11 files changed