Add an ASSERT about cross-world wrapper leakage into ScriptValue

Source/web/WebPluginContainerImpl.cpp

Index: Source/web/WebPluginContainerImpl.cpp
diff --git a/Source/web/WebPluginContainerImpl.cpp b/Source/web/WebPluginContainerImpl.cpp
index e3881dca90fa35233aa035f62cf71710b12d1d3d..27e70b3fcf48d838c7cf736819624d03f8fd0650 100644
--- a/Source/web/WebPluginContainerImpl.cpp
+++ b/Source/web/WebPluginContainerImpl.cpp
@@ -442,12 +442,13 @@ WebString WebPluginContainerImpl::executeScriptURL(const WebURL& url, bool popup
         kurl.string().substring(strlen("javascript:")));
 
     UserGestureIndicator gestureIndicator(popupsAllowed ? DefinitelyProcessingNewUserGesture : PossiblyProcessingUserGesture);
-    ScriptValue result = frame->script().executeScriptInMainWorldAndReturnValue(ScriptSourceCode(script));
+    v8::HandleScope handleScope(toIsolate(frame));
+    v8::Local<v8::Value> result = frame->script().executeScriptInMainWorldAndReturnValue(ScriptSourceCode(script));
 
     // Failure is reported as a null string.
-    String resultString;
-    result.toString(resultString);
-    return resultString;
+    if (result.IsEmpty() || !result->IsString())
+        return WebString();
+    return toCoreString(v8::Handle<v8::String>::Cast(result));
 }
 
 void WebPluginContainerImpl::loadFrameRequest(const WebURLRequest& request, const WebString& target, bool notifyNeeded, void* notifyData)