[gin] Allow interceptors to signal whether a they intercepted something

gin/interceptor.h

Index: gin/interceptor.h
diff --git a/gin/interceptor.h b/gin/interceptor.h
index 802929b83770bd316790f7d1c20f60b3f2380a7d..43cb3464fe97f5f3043b92adbb31540cd80fe461 100644
--- a/gin/interceptor.h
+++ b/gin/interceptor.h
@@ -25,7 +25,8 @@ class GIN_EXPORT NamedPropertyInterceptor {
 
   virtual v8::Local<v8::Value> GetNamedProperty(v8::Isolate* isolate,
                                                 const std::string& property);
-  virtual void SetNamedProperty(v8::Isolate* isolate,
+  // Return true if the set was interecepted.
+  virtual bool SetNamedProperty(v8::Isolate* isolate,
                                 const std::string& property,
                                 v8::Local<v8::Value> value);
   virtual std::vector<std::string> EnumerateNamedProperties(
@@ -45,7 +46,8 @@ class GIN_EXPORT IndexedPropertyInterceptor {
 
   virtual v8::Local<v8::Value> GetIndexedProperty(v8::Isolate* isolate,
                                                   uint32_t index);
-  virtual void SetIndexedProperty(v8::Isolate* isolate,
+  // Return true if the set was interecepted.
+  virtual bool SetIndexedProperty(v8::Isolate* isolate,
                                   uint32_t index,
                                   v8::Local<v8::Value> value);
   virtual std::vector<uint32_t> EnumerateIndexedProperties(