bindings: Throw a type error if converting an interface or callback function fails.

third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl

Index: third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl
diff --git a/third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl b/third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl
index 7ed4d7fc98e4581c2e4b3d1c61e923f1bff1b3c4..3edd411ba1ba2f3e956f2ccc9a14edfbb4d6304a 100644
--- a/third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl
+++ b/third_party/WebKit/Source/bindings/templates/interface.cpp.tmpl
@@ -892,7 +892,10 @@ v8::Local<v8::Object> {{v8_class}}::findInstanceInPrototypeChain(v8::Local<v8::V
 {##############################################################################}
 {% block native_value_traits %}
 {{cpp_class}}* NativeValueTraits<{{cpp_class}}>::nativeValue(v8::Isolate* isolate, v8::Local<v8::Value> value, ExceptionState& exceptionState) {
-  return {{v8_class}}::toImplWithTypeCheck(isolate, value);
+  {{cpp_class}}* nativeValue = {{v8_class}}::toImplWithTypeCheck(isolate, value);
+  if (!nativeValue)
+    exceptionState.throwTypeError("Unable to convert value to {{cpp_class}}.");

[Yuki, 2017/04/07 13:37:21]

nit: Can we use {{interface_name}} instead?
This is a user-visible message, so an internal C++ class' name is not preferred.

+  return nativeValue;
 }
 
 {% endblock %}