Adding support for simulcast and spatial layers into VideoQualityTest

This is a re-land of https://codereview.webrtc.org/1353263005/
which was reverted because of perf-regressions. Changes since that CL:

* Change LayerFilteringTransport to send a padding packet instead of
  dropping it for data that should be filtered out. This prevents
  confusion due to changed sequence numbers.

* Changed timing of stats poller thread in VideoAnalyzer. Startup was
  racy wrt initializion of send_stream_.

* Minor formatting issues.

PERF NOTE: This change will affect some performance numbers slightly.
In particular, {encode_frame_rate, encode_time_ms,
encode_usage_percent, media_bitrate_bps} will change due to timing
of the measurements.

BUG=
R=pbos@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1412233003

Cr-Commit-Position: refs/heads/master@{#10483}