BalancedDegradationSettings: add option to configure a min framerate diff.

If a framerate reduction (input fps - restricted fps) is less than the
configured diff, shorten interval to next qp check.

Bug: none
Change-Id: Ia0b9e0638e5ba75cdc20a1bb45bfcb7d858c5f89
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149040
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28880}

modules/video_coding/utility/quality_scaler_unittest.cc

@@ -36,9 +36,10 @@ class MockAdaptationObserver : public AdaptationObserverInterface {
     adapt_up_events_++;
     event.Set();
   }
-  void AdaptDown(AdaptReason r) override {
+  bool AdaptDown(AdaptReason r) override {
     adapt_down_events_++;
     event.Set();
+    return true;
   }
 
   rtc::Event event;