Add ability to configure quality scaler settings through field trial.

optional<int> min_frames: The minimum number frames to observe to make a
                          scaling decision.
Default: kMinFramesNeededToScale in quality_scaler.cc

optional<double> initial_scale_factor: The sample period scale factor.
Default: kSamplePeriodScaleFactor in quality_scaler.cc

optional<double> scale_factor: Option to use a reduced sampling interval when
                               last check did not result in an adaptation (if
                               unset the initial_scale_factor is used).

Bug: none
Change-Id: I3bb955d1f8d7d7d49bc118361614b5aa59605231
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135125
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27860}