iOS: Add improved BWE field trial.

webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm

Index: webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
index 6e4230de88d9e38dd65f1089aca30c0b78ae0dea..ff76133ab705b34df52e8857ebd605af3bd8c552 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -16,12 +16,20 @@
 
 #include "webrtc/system_wrappers/include/field_trial_default.h"
 
+static NSString * const kRTCEnableImprovedBitrateEstimateString =
+   @"WebRTC-ImprovedBitrateEstimate/Enabled/";
+
 static std::unique_ptr<char[]> gFieldTrialInitString;
 
 void RTCInitFieldTrials(RTCFieldTrialOptions options) {
   NSMutableString *fieldTrialInitString = [NSMutableString string];
   size_t len = fieldTrialInitString.length + 1;
   gFieldTrialInitString.reset(new char[len]);
+
+  if (options & RTCFieldTrialOptionsImprovedBitrateEstimate) {
+    [fieldTrialInitString appendString:kRTCEnableImprovedBitrateEstimateString];
+  }
+
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()
                               maxLength:len
                                encoding:NSUTF8StringEncoding]) {