RTCInboundRTPStreamStats.qpSum collected.

This was previously only collected for local tracks
(RTCOutboundRTPStreamStats.qpSum).

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcrtpstreamstats-qpsum

This CL also improves some testing in rtcstatscollector_unittest.cc.
Default and non-default values are tested in the same unittests,
removing the test that was specific to default-values, which was
otherwise code duplication.

BUG=webrtc:7065

Review-Url: https://codereview.webrtc.org/2675943002
Cr-Original-Commit-Position: refs/heads/master@{#16477}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cd195bea5e38a61e73b08f82445560d9136cd9da
4 files changed