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

third_party/WebKit/public/platform/WebRTCStats.h

Index: third_party/WebKit/public/platform/WebRTCStats.h
diff --git a/third_party/WebKit/public/platform/WebRTCStats.h b/third_party/WebKit/public/platform/WebRTCStats.h
index 40aa5c4e1e1894134770a12c9dd32094f53815c2..e7e67b4370b8a369f2023afbc9f3e3db8f93de48 100644
--- a/third_party/WebKit/public/platform/WebRTCStats.h
+++ b/third_party/WebKit/public/platform/WebRTCStats.h
@@ -48,6 +48,8 @@ class BLINK_PLATFORM_EXPORT WebRTCStatsReport {
   virtual std::unique_ptr<WebRTCStats> GetStats(WebString id) const = 0;
   // The next stats object, or null if the end has been reached.
   virtual std::unique_ptr<WebRTCStats> Next() = 0;
+  // The number of stats objects.
+  virtual size_t Size() const = 0;
 };
 
 class BLINK_PLATFORM_EXPORT WebRTCStats {