IDL: Add forEach() method to iterable<>/maplike<>/setlike<> interfaces

LayoutTests/http/tests/fetch/script-tests/headers.js

Index: LayoutTests/http/tests/fetch/script-tests/headers.js
diff --git a/LayoutTests/http/tests/fetch/script-tests/headers.js b/LayoutTests/http/tests/fetch/script-tests/headers.js
index 3ad8cb83acbc326024c2919b448776909a6b6a31..c458562a4a03473b6871e462e5290c8231031bdb 100644
--- a/LayoutTests/http/tests/fetch/script-tests/headers.js
+++ b/LayoutTests/http/tests/fetch/script-tests/headers.js
@@ -116,14 +116,26 @@ test(function() {
     }
 
     // 'entries()'
+    var entries = [];
     for (var header of headers.entries()) {
       var key = header[0], value = header[1];
       assert_not_equals(key, deleteKey.toLowerCase());
       assert_true(key in expectedValueMap);
       assert_equals(headers.get(key), expectedValueMap[key]);
       assert_equals(value, expectedValueMap[key]);
+      entries.push(header);
     }
 
+    // 'forEach()'
+    var thisObject = {};
+    headers.forEach(function (value, key, headersObject) {
+      var header = entries.shift();
+      assert_equals(key, header[0]);
+      assert_equals(value, header[1]);
+      assert_equals(thisObject, this);
+      assert_equals(headersObject, headers);
+    }, thisObject);
+
     // 'append()', 'getAll()'
     var allValues = headers.getAll('X-Fetch-Test');
     assert_equals(allValues.length, 1);