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

webrtc/examples/objc/AppRTCMobile/ARDAppClient.m

Index: webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
diff --git a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
index 1020621cc1bd2b91340963ac20609688aaacd8f6..ecf65286d8ad09a94f75ce1555bba2bedb348648 100644
--- a/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
+++ b/webrtc/examples/objc/AppRTCMobile/ARDAppClient.m
@@ -101,6 +101,7 @@ static int64_t const kARDAppClientRtcEventLogMaxSizeInBytes = 5e6;  // 5 MB.
 @implementation ARDAppClient {
   RTCFileLogger *_fileLogger;
   ARDTimerProxy *_statsTimer;
+  RTCMediaConstraints *_defaultMediaStreamConstraints;
 }
 
 @synthesize shouldGetStats = _shouldGetStats;
@@ -321,6 +322,10 @@ static int64_t const kARDAppClientRtcEventLogMaxSizeInBytes = 5e6;  // 5 MB.
 #endif
 }
 
+- (void)setDefaultMediaStreamConstraints:(RTCMediaConstraints *)mediaConstraints {
+  _defaultMediaStreamConstraints = mediaConstraints;
+}
+
 #pragma mark - ARDSignalingChannelDelegate
 
 - (void)channel:(id<ARDSignalingChannel>)channel
@@ -737,18 +742,14 @@ static int64_t const kARDAppClientRtcEventLogMaxSizeInBytes = 5e6;  // 5 MB.
    NSString *valueLevelControl = _shouldUseLevelControl ?
        kRTCMediaConstraintsValueTrue : kRTCMediaConstraintsValueFalse;
    NSDictionary *mandatoryConstraints = @{ kRTCMediaConstraintsLevelControl : valueLevelControl };
-   RTCMediaConstraints* constraints =
-       [[RTCMediaConstraints alloc]  initWithMandatoryConstraints:mandatoryConstraints
-                                              optionalConstraints:nil];
+   RTCMediaConstraints *constraints =
+       [[RTCMediaConstraints alloc] initWithMandatoryConstraints:mandatoryConstraints
+                                             optionalConstraints:nil];
    return constraints;
 }
 
 - (RTCMediaConstraints *)defaultMediaStreamConstraints {
-  RTCMediaConstraints* constraints =
-      [[RTCMediaConstraints alloc]
-          initWithMandatoryConstraints:nil
-                   optionalConstraints:nil];
-  return constraints;
+  return _defaultMediaStreamConstraints;
 }
 
 - (RTCMediaConstraints *)defaultAnswerConstraints {