Schedule HTTP requests to SPDY proxies via Resource Scheduler

Enable the experiment to schedule HTTP requests to SPDY proxies
via resource scheduler. The experiment has been running on Stable
M-67+ for some time. Results here: http://shortn/_xRuL16I5De,
and they show slight page load performance improvement.

With the experiment disabled, HTTP requests to SPDY proxies
would bypass scheduling enforced by resource scheduler, and
sunh requests would be immediately dispatched on the network.

With the experiment enabled (as well as with this CL), the
HTTP requests to SPDY proxies are also subject to scheduling.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I8e11e454addde53a6196d8af728bc0e5a4ecc789
Bug: 838253
Reviewed-on: https://chromium-review.googlesource.com/1137982
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575839}
7 files changed