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This CL allows a SpdySession to be created by passing a StreamSocket
to its constructor directly, in addition to keeping the previous
constructor, which takes a ClientSocketHandle. The new constructor
is intended for use where there's no socket pool below the session.

In particular, for H2 proxies, the SpdySessions are created below
the highest socket pool layer, at the ConnectJob layer. Once the socket
pools are flight, this will become the only socket pool layer, so
we'll only be dealing with a raw StreamSockets, rather than
ClientSocketHandles.

This is part of an effort to flatten the socket pools.
https://docs.google.com/document/d/1g0EA4iDqaDhNXA_mq-YK3SlSX-xRkoKvZetAQqdRrxM/edit

Change-Id: Ibe722bad9814879504757c03e8ad3c135d123971
Reviewed-on: https://chromium-review.googlesource.com/c/1453107
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629882}