wangyix: Can you take a look at this? it should be fairly straight forward to plumb the request priority down through the stream request and into the resolver.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a

commit 247ea64167abfb6e96d0baa3fbbcb99a74f4c01a
Author: Yixin Wang <wangyix@chromium.org>
Date: Wed Nov 15 01:15:50 2017

Use the request priority in QuicStreamFactory::Job's host resolution step instead of DEFAULT_PRIORITY

This adds a RequestPriority param to QuicStreamRequest::Request()

Bug:  768043 
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I9feb7f4d66eb971e6c66aa444317116c03dbdc2e
Reviewed-on: https://chromium-review.googlesource.com/767853
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Yixin Wang <wangyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516542}
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/http/http_proxy_client_socket_wrapper.cc
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/http/http_stream_factory_impl_job.cc
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/quic/chromium/quic_stream_factory.cc
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/quic/chromium/quic_stream_factory.h
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/quic/chromium/quic_stream_factory_fuzzer.cc
[modify] https://crrev.com/247ea64167abfb6e96d0baa3fbbcb99a74f4c01a/net/quic/chromium/quic_stream_factory_test.cc