Add setting to AppRTCMobile for iOS, that can change capture resolution.

webrtc/examples/objc/AppRTCMobile/ARDAppClient.h

Index: webrtc/examples/objc/AppRTCMobile/ARDAppClient.h
diff --git a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.h b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.h
index 2186abe6936fee7241fca537d33020d4f2ea002c..4f6f25fea4173ca129f5e17d0ce7e75db2374cfd 100644
--- a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.h
+++ b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.h
@@ -23,6 +23,8 @@ typedef NS_ENUM(NSInteger, ARDAppClientState) {
 };
 
 @class ARDAppClient;
+@class RTCMediaConstraints;
+
 // The delegate is informed of pertinent events and will be called on the
 // main queue.
 @protocol ARDAppClientDelegate <NSObject>
@@ -56,11 +58,13 @@ typedef NS_ENUM(NSInteger, ARDAppClientState) {
 @property(nonatomic, assign) BOOL shouldGetStats;
 @property(nonatomic, readonly) ARDAppClientState state;
 @property(nonatomic, weak) id<ARDAppClientDelegate> delegate;
-
 // Convenience constructor since all expected use cases will need a delegate
 // in order to receive remote tracks.
 - (instancetype)initWithDelegate:(id<ARDAppClientDelegate>)delegate;
 
+// Sets default media stream constraints.
+- (void)setDefaultMediaStreamConstraints:(RTCMediaConstraints *)mediaConstraints;
+
 // Establishes a connection with the AppRTC servers for the given room id.
 // If |isLoopback| is true, the call will connect to itself.
 // If |isAudioOnly| is true, video will be disabled for the call.