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

webrtc/examples/objc/AppRTCMobile/ios/ARDMediaConstraintsModel.h

+/*
+ *  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 <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+/**
+ * Model class for user defined media constraints.
+ *
+ * Currently used for streaming media constraints only.
+ * In future audio media constraints support can be added as well.
+ * Offers list of avaliable video resolutions that can construct streaming media constraint.
+ * Exposes methods for reading and storing media constraints from persistent store.
+ * Also translates current user defined media constraint into RTCMediaConstraints
+ * dictionary.
+ */
+@interface ARDMediaConstraintsModel : NSObject
+
+/**
+ * Returns array of available capture resoultions.
+ *
+ * The capture resolutions are represented as strings in the following format
+ * [width]x[height]
+ */
+- (NSArray<NSString *> *)availableVideoResoultionsMediaConstraints;
+
+/**
+ * Returns current video resolution media constraint string.
+ * If no constraint is in store, default value of 640x480 is returned.
+ * When defaulting to value, the default is saved in store for consistency reasons.
+ */
+- (NSString *)currentVideoResoultionConstraintFromStore;
+
+/**
+ * Stores the provided video resolution media constraint string into the store.
+ *
+ * If the provided constraint is no part of the available video resolutions
+ * the store operation will not be executed and NO will be returned.
+ * @param constraint the string to be stored.
+ * @return YES/NO depending on success.
+ */
+- (BOOL)storeVideoResoultionConstraint:(NSString *)constraint;
+
+/**
+ * Converts the current media constraints from store into dictionary with RTCMediaConstraints
+ * values.
+ *
+ * @return NSDictionary with RTC width and height parameters
+ */
+- (nullable NSDictionary *)currentMediaConstraintFromStoreAsRTCDictionary;
+
+@end
+NS_ASSUME_NONNULL_END