Enable iOS AppRTCDemo send side BWE.

BUG=

Review URL: https://codereview.webrtc.org/1757173002

Cr-Commit-Position: refs/heads/master@{#11865}

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

@@ -10,6 +10,7 @@
 
 #import "ARDAppDelegate.h"
 
+#import "webrtc/base/objc/RTCFieldTrials.h"
 #import "webrtc/base/objc/RTCTracing.h"
 #import "RTCLogging.h"
 #import "RTCPeerConnectionFactory.h"
@@ -24,6 +25,7 @@
 
 - (BOOL)application:(UIApplication *)application
     didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
+  RTCInitFieldTrials(RTCFieldTrialOptionsSendSideBwe);
   [RTCPeerConnectionFactory initializeSSL];
   RTCSetupInternalTracer();
   _window =  [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];