Add field trials to configure the backoff factor and the trendline window of the BWE.

These field trials can be set with a string similar to:
WebRTC-BweWindowSizeInPackets/Enabled-150/WebRTC-BweBackOffFactor/Enabled-0.95/

BweWindowSizeInPackets
Number of packets which the delay-based BWE window is based on. A larger value means lower delay-sensitivity.
Default in WebRTC: 20
Reasonable values for streaming: 50-150

BweBackOffFactor
How far the BWE will back off when the delay increases. A value closer to 1.0 means smaller back-off.
Range: > 0.0, < 1.0
Default in WebRTC: 0.85
Reasonable values for streaming: 0.85-0.95

Bug: webrtc:8212
Change-Id: I61f0883788b689847a43273b63cef663042f4d42
Reviewed-on: https://webrtc-review.googlesource.com/6764
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20172}
diff --git a/modules/remote_bitrate_estimator/BUILD.gn b/modules/remote_bitrate_estimator/BUILD.gn
index 4784a83..f6fe97d 100644
--- a/modules/remote_bitrate_estimator/BUILD.gn
+++ b/modules/remote_bitrate_estimator/BUILD.gn
@@ -170,6 +170,7 @@
       ":bwe_simulator_lib",
       ":remote_bitrate_estimator",
       "../../rtc_base:rtc_base_approved",
+      "../../test:field_trial",
       "../../test:test_support",
     ]
     if (!build_with_chromium && is_clang) {