Make quality scaler downscale faster.

Include dropped frames by the encoder in the frame drop percentage.

To react faster at low framerates:
- Use ExpFilter instead of MovingAverage to filter QP values.
- Reduce sampling interval while waiting for minimum number of needed frames (when not in fast rampup mode).

A separate slower ExpFilter is used for upscaling.

Bug: webrtc:9169
Change-Id: If7ff6c3bd4201fda2da67125889838fe96ce7061
Reviewed-on: https://webrtc-review.googlesource.com/70761
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23014}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index 619f15e..b2fd91d 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -243,6 +243,7 @@
     "../../rtc_base:rtc_numerics",
     "../../rtc_base:rtc_task_queue",
     "../../rtc_base:sequenced_task_checker",
+    "../../rtc_base/experiments:quality_scaling_experiment",
     "../../system_wrappers",
     "../rtp_rtcp:rtp_rtcp_format",
   ]