iOS: Fix 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 ff76133ab705b34df52e8857ebd605af3bd8c552..2c538cdb46d5c5b5104f6d072a7d663c6f702581 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -23,13 +23,11 @@ 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];
   }
-
+  size_t len = fieldTrialInitString.length + 1;
+  gFieldTrialInitString.reset(new char[len]);
   if (![fieldTrialInitString getCString:gFieldTrialInitString.get()
                               maxLength:len
                                encoding:NSUTF8StringEncoding]) {