Implements peer connection stats measurements in WebRTC call test.

Implements peer connection stats measurements in WebRTC call test.

The telemetry test for WebRTC has so far been measuring CPU and memory.
Since there's a stats API for WebRTC (see http://goo.gl/lfQKzE), we try
to use that do extract interesting properties about the call. The
properties we look at are such that we expect them to remain stable, so
a sudden change in any of them will warrant investigation.

This implementation is based on a trick where we instrument the
constructor for webkitRTCPeerConnection. Whenever a peer connection is
constructed, we attach a polling function which periodically polls the
connection for new stats (that's how the stats API works). Then we
make it easy for a measurement to extract this data, aggregate it into
time series and report it through Telemetry.

BUG=412787

Review URL: https://codereview.chromium.org/561803002

Cr-Commit-Position: refs/heads/master@{#300249}