Record stream IDs in Http2PushPromiseIndex.

Stop using SpdySession instances in Http2PushPromiseIndexTests.  This
makes tests much simpler.

Rename SpdySessionKey::Equals() to SpdySessionKey::operator==() so that
it can be tested by gMock.  This conforms to the style guide "prefer to
define == [...] rather than Equals()", see
https://google.github.io/styleguide/cppguide.html#Operator_Overloading

Define struct UnclaimedPushedStream instead of using std::pair or
std::tuple.  Named members are much nicer than first, second, or get<>.

Define CompareByUrl in conformance with the same section of the style
guide "if your type doesn't have a natural ordering [...] use a custom
comparator rather than overloading <".

The next CL will merge SpdySession::UnclaimedPushedStreamContainer into
Http2PushPromiseIndex.  For this, two sets of UnclaimedPushedStream
entries will need to be maintained with identical
contents but different ordering.  The other will be done by
CompareByDelegate and will allow efficient lookup of entries for a given
Delegate.

Bug: 776415, 791054
Change-Id: I0c1912e9932b102e17528f9319163232860a8813
Reviewed-on: https://chromium-review.googlesource.com/809187
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522192}
9 files changed