Use different restrictions of acked bitrate lag depending on operating point.

webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc

Index: webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc b/webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc
index 530453d74cc1612c105de25bb7cc0280d261c53a..8a6eb06f2df18080ea53772faaad34d4fec70220 100644
--- a/webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc
+++ b/webrtc/modules/remote_bitrate_estimator/aimd_rate_control.cc
@@ -229,11 +229,14 @@ uint32_t AimdRateControl::ChangeBitrate(uint32_t current_bitrate_bps,
     default:
       assert(false);
   }
-  if ((incoming_bitrate_bps > 100000 || current_bitrate_bps > 150000) &&
-      current_bitrate_bps > 1.5 * incoming_bitrate_bps) {
-    // Allow changing the bit rate if we are operating at very low rates
-    // Don't change the bit rate if the send side is too far off
-    current_bitrate_bps = current_bitrate_bps_;
+  // Don't change the bit rate if the send side is too far off.
+  // We allow a bit more lag at very low rates to not too easily get stuck if
+  // the encoder produces uneven outputs.
+  const uint32_t max_bitrate_bps =
+      static_cast<uint32_t>(1.5f * incoming_bitrate_bps) + 10000;
+  if (current_bitrate_bps > current_bitrate_bps_ &&
+      current_bitrate_bps > max_bitrate_bps) {
+    current_bitrate_bps = std::max(current_bitrate_bps_, max_bitrate_bps);
     time_last_bitrate_change_ = now_ms;
   }
   return current_bitrate_bps;