This would make it possible to gather stats on multiple threads, store
the results in multiple reports and to merge the results.
Added rtcstatsreport_unittest.cc, moving a RTCStatsReport-related test
from rtcstats_unittest.cc. Added more unittests covering the order of
stats and TakeMembersFrom.
Also changed RTCStatsReport[] to RTCStatsReport::Get to avoid
confusion with other usages of the [] operator.
BUG=chromium:627816
NOTRY=True
Review-Url: https://codereview.webrtc.org/2278433003
Cr-Commit-Position: refs/heads/master@{#13957}
6d183ac27a