Add a histogram for Vary response header in pushed HTTP/2 streams.

This is to assess if pushed responses with Vary header could be rejected
(if proportion is very-very low).  See public discussion at
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/5_aP_stqndw.

Also add unittests.

Bug: 554220
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I8c5c95fc12da056fed7f837c79419e969f991a35
Reviewed-on: https://chromium-review.googlesource.com/755093
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516094}
5 files changed