iOS: Add improved BWE field trial.

BUG=webrtc:6664
NOTRY=True

Review-Url: https://codereview.webrtc.org/2482593002
Cr-Commit-Position: refs/heads/master@{#14958}

webrtc/examples/objc/AppRTCMobile/ios/ARDAppDelegate.m

@@ -25,7 +25,7 @@
 
 - (BOOL)application:(UIApplication *)application
     didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
-  RTCInitFieldTrials(RTCFieldTrialOptionsSendSideBwe);
+  RTCInitFieldTrials(RTCFieldTrialOptionsImprovedBitrateEstimate);
   RTCInitializeSSL();
   RTCSetupInternalTracer();
   _window =  [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

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]) {

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCFieldTrials.h

@@ -14,7 +14,7 @@
 
 typedef NS_OPTIONS(NSUInteger, RTCFieldTrialOptions) {
   RTCFieldTrialOptionsNone = 0,
-  RTCFieldTrialOptionsSendSideBwe = 1 << 0,
+  RTCFieldTrialOptionsImprovedBitrateEstimate = 1 << 0,
 };
 
 /** Must be called before any other call into WebRTC. See: