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/Classes/PeerConnection/RTCIntervalRange.mm

@@ -0,0 +1,50 @@
+/*
+ *  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 "RTCIntervalRange+Private.h"
+
+#include "webrtc/rtc_base/checks.h"
+
+@implementation RTCIntervalRange
+
+@synthesize min = _min;
+@synthesize max = _max;
+
+- (instancetype)init {
+  return [self initWithMin:0 max:0];
+}
+
+- (instancetype)initWithMin:(NSInteger)min
+                        max:(NSInteger)max {
+  RTC_DCHECK_LE(min, max);
+  if (self = [super init]) {
+    _min = min;
+    _max = max;
+  }
+  return self;
+}
+
+- (instancetype)initWithNativeIntervalRange:(const rtc::IntervalRange &)config {
+  return [self initWithMin:config.min() max:config.max()];
+}
+
+- (NSString *)description {
+  return [NSString stringWithFormat:@"[%ld, %ld]", (long)_min, (long)_max];
+}
+
+#pragma mark - Private
+
+- (std::unique_ptr<rtc::IntervalRange>)nativeIntervalRange {
+  std::unique_ptr<rtc::IntervalRange> nativeIntervalRange(
+      new rtc::IntervalRange((int)_min, (int)_max));
+  return nativeIntervalRange;
+}
+
+@end