Create Http2PushPromiseIndex class.

Factor out SpdySessionPool::unclaimed_pushed_streams_ into its own
class, similar to QuicClientPushPromiseIndex.  This class will handle
async push promise validation after some future CLs.

No functional change.

Bug: 554220
Change-Id: If42d077c6d258fc419b822db7511623904629aae
Reviewed-on: https://chromium-review.googlesource.com/692935
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505647}
6 files changed