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This CL is in preparation for https://crrev.com/c/734223.

Create Http2PushPromiseIndex::Delegate and thus remove circular dependency
between Http2PushPromiseIndex and SpdySession.

Also, change the internal container type of Http2PushPromiseIndex from
map<vector> to set<pair>.  Later, when
SpdySession::UnclaimedStreamContainer is torn out from SpdySession and
is merged into Http2PushPromiseIndex, there will be multile set<tuple>
containers, storing identical data but sorted by different keys, kept in
sync internally (because fast lookup will be required both by GURL and
by SpdySession*).  set<tuple> will be a lot simpler for this purpose
than map<vector>.

Also, unlike in draft https://crrev.com/c/734223, keep only Delegate raw
pointers and use those to generate the WeakPtr<SpdySession>, instead of
keeping a copy of WeakPtr in addition.  Again, this is to simplify
things.

Bug: 791055
Change-Id: I8c9fcd77c85f3801eb24ca171c66d18ea027965b
Reviewed-on: https://chromium-review.googlesource.com/797438
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521156}