Never include unsupported event attributes into attributeNamesWithInt…

…erfaceName

This is a small refactoring of #49473

Instead of skipping unsupported attributes in the for loop, we filter
them out when building the list.

Technically, interfaceName is no longer used in the for loop, but we
keep it in case that's needed for future setAttribute/setAttributeNS
tests.

trusted-types/TrustedTypePolicyFactory-getAttributeType-event-handler-content-attributes.tentative.html

@@ -11,6 +11,20 @@
 <script>
 promise_setup(async function() {
   let attributeNamesWithInterfaceName = [];
+  function isAttributeImplemented(interfaceName, name) {
+    switch (interfaceName) {
+      case 'GlobalEventHandlers':
+        return name in HTMLElement.prototype;
+      case 'WindowEventHandlers':
+        return name in HTMLBodyElement.prototype;
+      case 'HTMLMediaElement':
+        return name in HTMLMediaElement.prototype;
+      case 'SVGAnimationElement':
+        return name in SVGAnimationElement.prototype;
+      default:
+       throw "Unknown interface";
+    }
+  }
   function addOnAttributes(IDL, interfaceName) {
     // Parsing the whole IDL file is slow, so use a small regexp to extract only
     // the part that is relevant for this test.
@@ -19,7 +33,8 @@
     parsedIDL.find(idl => idl.name === interfaceName)
       .members.map(member => member.name)
       .filter(name => name.length >= 3 && name.startsWith("on") &&
-              !name.startsWith("onwebkit"))
+              !name.startsWith("onwebkit") &&
+              isAttributeImplemented(interfaceName, name))
       .forEach(name => attributeNamesWithInterfaceName.push({name, interfaceName}));
   }
 
@@ -39,20 +54,6 @@
   addOnAttributes(svgAnimationsIDL, "SVGAnimationElement");
 
   for (const attributeNameWithInterfaceName of attributeNamesWithInterfaceName) {
-    // Skip attributes that are not implemented in the browser.
-    if (attributeNameWithInterfaceName.interfaceName === 'GlobalEventHandlers' && !(attributeNameWithInterfaceName.name in HTMLElement.prototype)) {
-      continue;
-    }
-    if (attributeNameWithInterfaceName.interfaceName === 'WindowEventHandlers' && !(attributeNameWithInterfaceName.name in HTMLBodyElement.prototype)) {
-      continue;
-    }
-    if (attributeNameWithInterfaceName.interfaceName === 'HTMLMediaElement' && !(attributeNameWithInterfaceName.name in HTMLMediaElement.prototype)) {
-      continue;
-    }
-    if (attributeNameWithInterfaceName.interfaceName === 'SVGAnimationElement' && !(attributeNameWithInterfaceName.name in SVGAnimationElement.prototype)) {
-      continue;
-    }
-
     promise_test(async () => {
       NSURI_ARRAY.forEach(attrNs => {
         assert_equals(trustedTypes.getAttributeType(