[Bindings] Make maplike<> and setlike<> imply a readonly size attribute.

content/renderer/media/webrtc/rtc_stats.cc

Index: content/renderer/media/webrtc/rtc_stats.cc
diff --git a/content/renderer/media/webrtc/rtc_stats.cc b/content/renderer/media/webrtc/rtc_stats.cc
index 46b82c500270b0776a74bd3fae6b8deab05565bd..0092c6275290dd29a50f12ce34bded8ed611e32a 100644
--- a/content/renderer/media/webrtc/rtc_stats.cc
+++ b/content/renderer/media/webrtc/rtc_stats.cc
@@ -95,6 +95,10 @@ std::unique_ptr<blink::WebRTCStats> RTCStatsReport::Next() {
   return std::unique_ptr<blink::WebRTCStats>();
 }
 
+size_t RTCStatsReport::Size() const {
+  return stats_report_->size();
+}
+
 RTCStats::RTCStats(
     const scoped_refptr<const webrtc::RTCStatsReport>& stats_owner,
     const webrtc::RTCStats* stats)