QualityScaler: Add option to try fast adapt down at start up based on initial bw estimates.

optional<int> initial_bitrate_interval_ms: time interval since start of call
where fast adapt down is allowed.
optional<double> initial_bitrate_factor: try fast adapt down if bw estimate is
below initial bitrate * factor.

Bug: none
Change-Id: I63e1fdaac6556d8e9a961a42e11c925f9ecb9771
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147725
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28753}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 7d83ffe..6221053 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -213,6 +213,7 @@
     "../rtc_base/experiments:alr_experiment",
     "../rtc_base/experiments:balanced_degradation_settings",
     "../rtc_base/experiments:field_trial_parser",
+    "../rtc_base/experiments:quality_scaler_settings",
     "../rtc_base/experiments:quality_scaling_experiment",
     "../rtc_base/experiments:rate_control_settings",
     "../rtc_base/synchronization:sequence_checker",