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Extract QuicServerSessionBasePeer to its own file so that GfeQuicServerSessionPeer does not need to be a friend of QuicServerSessionBase.

Change-Id: I8d0b2743f903f3fdfaa06346a9f006fccf78e622
Reviewed-on: https://chromium-review.googlesource.com/1148688Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577672}