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/examples/objc/AppRTCDemo/ios/ARDStatsView.m

@@ -10,7 +10,7 @@
 
 #import "ARDStatsView.h"
 
-#import "WebRTC/RTCStatsReport.h"
+#import "WebRTC/RTCLegacyStatsReport.h"
 
 #import "ARDStatsBuilder.h"
 
@@ -35,7 +35,7 @@
 }
 
 - (void)setStats:(NSArray *)stats {
-  for (RTCStatsReport *report in stats) {
+  for (RTCLegacyStatsReport *report in stats) {
     [_statsBuilder parseStatsReport:report];
   }
   _statsLabel.text = _statsBuilder.statsString;