Socket Pools Refactor 16 (1): Remove SSLClientSocketPool.

Remove the SSLClientSocketPool - use TransportConnectJobs instead.

This CL allows TransportClientSocketPools to stack on top of other
TransportClientSocketPools, when being used for SSLClientSockets.

It also leaves ssl_client_socket_pool_unittests in place, those just
use TransportClientSocketPools now, and will be moved in future CLs.

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

Bug: 472729
Change-Id: I473f28136347ce6e2bd17c5da0bcf4d7ac648072
Reviewed-on: https://chromium-review.googlesource.com/c/1446244
Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628479}