Add ObjC API for getting native histograms.

BUG=

Review-Url: https://codereview.webrtc.org/2036773003
Cr-Commit-Position: refs/heads/master@{#13064}

webrtc/sdk/objc/Framework/Classes/RTCMetricsSampleInfo+Private.h

@@ -0,0 +1,25 @@
+/*
+ *  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 "WebRTC/RTCMetricsSampleInfo.h"
+
+#include "webrtc/system_wrappers/include/metrics_default.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCMetricsSampleInfo ()
+
+/** Initialize an RTCMetricsSampleInfo object from native SampleInfo. */
+- (instancetype)initWithNativeSampleInfo:
+    (const webrtc::metrics::SampleInfo &)info;
+
+@end
+
+NS_ASSUME_NONNULL_END