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

third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h

Index: third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h
diff --git a/third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h b/third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h
index 226b6ed3be41a10f9d65c35ff1a693a05ee1d0b0..b2ab22f1eb44ceeda2503d4d6b5716c6235fdef5 100644
--- a/third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h
+++ b/third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h
@@ -25,6 +25,8 @@ class RTCStatsReport final : public GarbageCollectedFinalized<RTCStatsReport>,
  public:
   RTCStatsReport(std::unique_ptr<WebRTCStatsReport>);
 
+  size_t size() const { return report_->Size(); }
+
   // Maplike<String, v8::Local<v8::Value>>
   PairIterable<String, v8::Local<v8::Value>>::IterationSource* StartIteration(
       ScriptState*,