Renamed RTCStatsReport to RTCLegacyStatsReport in objc files.

This is to avoid a naming conflict with webrtc::RTCStatsReport that is
surfaced if you try to include it in peerconnectioninterface.h.

Background: The current stats is very much non-spec-compliant. A new
stats collection API is underway that is meant to be spec-compliant.
Some classes in Chromium and webrtc/sdk/objc have spec-compliant names
but non-spec-compliant behavior. These are being renamed to "Legacy" so
that new spec-compliant classes can be added with the correct names.

BUG=chromium:627816
TBR=tkchin@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2313943002
Cr-Commit-Position: refs/heads/master@{#14150}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

@@ -14,13 +14,13 @@
 #import "RTCConfiguration+Private.h"
 #import "RTCDataChannel+Private.h"
 #import "RTCIceCandidate+Private.h"
+#import "RTCLegacyStatsReport+Private.h"
 #import "RTCMediaConstraints+Private.h"
 #import "RTCMediaStream+Private.h"
 #import "RTCPeerConnectionFactory+Private.h"
 #import "RTCRtpReceiver+Private.h"
 #import "RTCRtpSender+Private.h"
 #import "RTCSessionDescription+Private.h"
-#import "RTCStatsReport+Private.h"
 #import "WebRTC/RTCLogging.h"
 
 #include <memory>