Update API for Objective-C RTCStats.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1540113002 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11207}
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index 482e62b..e5a70e9 100644
--- a/webrtc/api/api.gyp
+++ b/webrtc/api/api.gyp
@@ -31,6 +31,9 @@
             'objc/RTCSessionDescription+Private.h',
             'objc/RTCSessionDescription.h',
             'objc/RTCSessionDescription.mm',
+            'objc/RTCStatsReport+Private.h',
+            'objc/RTCStatsReport.h',
+            'objc/RTCStatsReport.mm',
             'objc/RTCVideoFrame+Private.h',
             'objc/RTCVideoFrame.h',
             'objc/RTCVideoFrame.mm',