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/WebRTC.h

@@ -25,6 +25,8 @@
 #import <WebRTC/RTCMediaConstraints.h>
 #import <WebRTC/RTCMediaStream.h>
 #import <WebRTC/RTCMediaStreamTrack.h>
+#import <WebRTC/RTCMetrics.h>
+#import <WebRTC/RTCMetricsSampleInfo.h>
 #import <WebRTC/RTCPeerConnection.h>
 #import <WebRTC/RTCPeerConnectionFactory.h>
 #import <WebRTC/RTCRtpCodecParameters.h>