Don't pool HTTP2 sessions used for tunnels and direct connections.

Pooling sockets from independent socket pools together breaks layering
assumptions.  A later point in time, these pools will also need to
wrap lower layer objects than ClientSocketHandles.

Bug: 895562
Change-Id: I8235f244048475bcb64bf48b3e15309e8607e85e
Reviewed-on: https://chromium-review.googlesource.com/c/1362192
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615585}
21 files changed