Make CustomEvent#initCustomEvent match the spec

third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp

Index: third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp b/third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp
index 285eb9f56458a99d4c569b069470432e35d20ddd..3931195bc0e5f2d2099732e922e1d3b5fdd5d63a 100644
--- a/third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/custom/V8CustomEventCustom.cpp
@@ -106,13 +106,15 @@ void V8CustomEvent::constructorCustom(
 void V8CustomEvent::initCustomEventMethodEpilogueCustom(
     const v8::FunctionCallbackInfo<v8::Value>& info,
     CustomEvent* impl) {
-  ASSERT(info.Length() >= 3);
   if (impl->isBeingDispatched())
     return;
-  v8::Local<v8::Value> detail = info[3];
-  if (!detail.IsEmpty())
-    storeDetail(ScriptState::current(info.GetIsolate()), impl, info.Holder(),
-                detail);
+  if (info.Length() >= 4) {
+    v8::Local<v8::Value> detail = info[3];
+    if (!detail.IsEmpty()) {
+      storeDetail(ScriptState::current(info.GetIsolate()), impl, info.Holder(),
+                  detail);
+    }
+  }
 }
 
 void V8CustomEvent::detailAttributeGetterCustom(