Allow a preconnecting job to a HTTP2 proxy server based on privacy mode

Currently, one preconnect job is allowed in-flight for one proxy server.
This CL changes it to allow one in-flight preconnect job for one
(proxy server, privacy mode) tuple. This is needed since requests that
use different privacy modes cannot share the same connection.

BUG=667471
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2648593002
Cr-Commit-Position: refs/heads/master@{#445392}
5 files changed