In bwe tests Collect receive statistic using GetReportBlocks collector
instead of GetActiveStatistcian

BUG=webrtc:8016

Review-Url: https://codereview.webrtc.org/2996883002
Cr-Original-Commit-Position: refs/heads/master@{#19685}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7eb938d4c065014a6d33a4258b6e85e28de28359