Throttle HttpStreamFactoryImpl::Job to HTTP/2-supported servers

If there are multiple HttpStreamFactoryImpl::Jobs to the same HTTP/2-
supported server, throttle the socket connection establishments to
allow the first one to go through and then the rest. This is to reduce the number
of unnecessary HTTP/2 socket connections.

Design doc:
https://docs.google.com/document/d/1XeAOPduGVv95_U371bmlTgg8Parp_D3QK9aEeDoMSOo/edit?usp=sharing

BUG=718576

Review-Url: https://codereview.chromium.org/2932513004
Cr-Commit-Position: refs/heads/master@{#479721}

• net/http/http_stream_factory_impl_job.cc[diff]
• net/http/http_stream_factory_impl_job.h[diff]
• net/http/http_stream_factory_impl_job_controller_unittest.cc[diff]
• net/spdy/chromium/spdy_session_pool.cc[diff]
• net/spdy/chromium/spdy_session_pool.h[diff]