Reland: Store |detail| as a hidden value of custom event wrappers

third_party/WebKit/Source/core/events/CustomEvent.idl

Index: third_party/WebKit/Source/core/events/CustomEvent.idl
diff --git a/third_party/WebKit/Source/core/events/CustomEvent.idl b/third_party/WebKit/Source/core/events/CustomEvent.idl
index 5e6d7abd36049c1f94cae0be83e1847462b91137..ca1e27ff5cce78b37620ea5b58bf3d6db94cb977 100644
--- a/third_party/WebKit/Source/core/events/CustomEvent.idl
+++ b/third_party/WebKit/Source/core/events/CustomEvent.idl
@@ -26,10 +26,12 @@
 // https://dom.spec.whatwg.org/#interface-customevent
 
 [
-    Constructor(DOMString type, optional CustomEventInit eventInitDict),
+    // TODO(bashi): Don't use CustomConstructor. Constructor should be:
+    // Constructor(DOMString type, optional CustomEventInit eventInitDict),
+    CustomConstructor,
     Exposed=(Window,Worker),
 ] interface CustomEvent : Event {
     [Custom=Getter] readonly attribute any detail;
 
-    [Measure] void initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
+    [Measure,Custom=(CallEpilogue)] void initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
 };