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

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

+/*
+ *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import "ARDMediaConstraintsSettingsStore.h"
+
+static NSString *const kUserDefaultsMediaConstraintsKey =
+    @"rtc_video_resolution_media_constraints_key";
+
+NS_ASSUME_NONNULL_BEGIN
+@implementation ARDMediaConstraintsSettingsStore
+
+- (nullable NSString *)videoResolutionConstraintsSetting {
+  return [[NSUserDefaults standardUserDefaults] objectForKey:kUserDefaultsMediaConstraintsKey];
+}
+
+- (void)setVideoResolutionConstraintsSetting:(NSString *)constraintsString {
+  [[NSUserDefaults standardUserDefaults] setObject:constraintsString
+                                            forKey:kUserDefaultsMediaConstraintsKey];
+}
+
+@end
+NS_ASSUME_NONNULL_END