lib: define Event.isTrusted in the prototype

Don't conform to the spec with isTrusted. The spec defines it as
`LegacyUnforgeable` but defining it in the constructor has a big
performance impact and the property doesn't seem to be useful outside of
browsers.

Refs: nodejs/performance#32

lib/internal/event_target.js

@@ -119,8 +119,6 @@ class Event {
       isTrustedSet.add(this);
     }
 
-    // isTrusted is special (LegacyUnforgeable)
-    ObjectDefineProperty(this, 'isTrusted', isTrustedDescriptor);
     this[kTarget] = null;
     this[kIsBeingDispatched] = false;
   }
@@ -343,6 +341,11 @@ ObjectDefineProperties(
     eventPhase: kEnumerableProperty,
     cancelBubble: kEnumerableProperty,
     stopPropagation: kEnumerableProperty,
+    // Don't conform to the spec with isTrusted. The spec defines it as
+    // LegacyUnforgeable but defining it in the constructor has a big
+    // performance impact and the property doesn't seem to be useful outside of
+    // browsers.
+    isTrusted: isTrustedDescriptor,
   });
 
 function isCustomEvent(value) {