RTCBrowserType.supportsSimulcast(). Add TODOs.

This is only temporary, since we suspect that the calculation for the
receive direction is incorrect.

Cleans up code. Paces the increase of connection quality. Take ramp-up
into account. Uses sending bitrate by default whenever possible (with
and without simulcast, as long as we know the sending bitrate and
resolution).

Updates the packet loss based calculation (used whenever we don't know
the sending bitrate and input resolution) with hard-coded thresholds,
so that it doesn't scale linearly (15% packet loss doesn't yield 85%
connection quality).