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/Headers/WebRTC/RTCMetrics.h

@@ -0,0 +1,23 @@
+/*
+ *  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>
+
+#import <WebRTC/RTCMacros.h>
+#import <WebRTC/RTCMetricsSampleInfo.h>
+
+/**
+ * Enables gathering of metrics (which can be fetched with
+ * RTCGetAndResetMetrics). Must be called before any other call into WebRTC.
+ */
+RTC_EXTERN void RTCEnableMetrics();
+
+/** Gets and clears native histograms. */
+RTC_EXTERN NSArray<RTCMetricsSampleInfo *> *RTCGetAndResetMetrics();