Plumb NQP to context and to http_proxy_client_socket_pool

Also, refactor http_proxy_client_socket_pool.cc so that proxy
connection timeout is determined by a separate method.

The next CL would use Network Quality Provider (NQP) to determine the
proxy connection timeout in http_proxy_client_socket_pool.

BUG=704339

Review-Url: https://codereview.chromium.org/2899313006
Cr-Commit-Position: refs/heads/master@{#478159}
13 files changed