Reinstate "iOS - Add iceRegatherIntervalRange."

This reverts commit 93adc3209b5ff10adaba54d5eab6b53bc2780685.

Reverted originally because it depended on a CL which was reverted.
That CL has been reinstated in:

https: //chromium-review.googlesource.com/#/c/572070/
Bug: webrtc:7969
Change-Id: I608bbeaaba02e84908433c8260cf236df0307a97
Reviewed-on: https://chromium-review.googlesource.com/572405
Reviewed-by: Zeke Chin <tkchin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19035}

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCIntervalRange.h

@@ -0,0 +1,28 @@
+/*
+ *  Copyright 2017 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
+
+@interface RTCIntervalRange : NSObject
+
+@property(nonatomic, readonly) NSInteger min;
+@property(nonatomic, readonly) NSInteger max;
+
+- (instancetype)init;
+- (instancetype)initWithMin:(NSInteger)min
+                        max:(NSInteger)max
+    NS_DESIGNATED_INITIALIZER;
+
+@end
+
+NS_ASSUME_NONNULL_END
+