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-Commit-Position: refs/heads/master@{#16477}