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

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

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 test(function() {
     function size(headers) {
       var count = 0;
       for (var header of headers) {
         ++count;
       }
@@ skipping 98 matching lines @@
     var expectedKeyMap = {};
     for (var key in expectedValueMap)
       expectedKeyMap[expectedValueMap[key]] = key;
     for (var value of headers.values()) {
       assert_true(value in expectedKeyMap);
       var key = expectedKeyMap[value];
       assert_not_equals(key, deleteKey.toLowerCase());
     }
 
     // '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_true(thisObject === this);
+      assert_true(headersObject === headers);
+    }, thisObject);
+
     // 'append()', 'getAll()'
     var allValues = headers.getAll('X-Fetch-Test');
     assert_equals(allValues.length, 1);
     assert_equals(size(headers), 4);
     headers.append('X-FETCH-TEST', 'response test field - append');
     assert_equals(size(headers), 5, 'headers size should increase by 1.');
     assert_equals(headers.get('X-FETCH-Test'),
                   'response test field - updated',
                   'the value of the first header added should be returned.');
     allValues = headers.getAll('X-FETch-TEST');
@@ skipping 106 matching lines @@
                   'new Headers with a sequence with less than two strings ' +
                   'should throw');
     assert_throws({name: 'TypeError'},
                   function() { var headers = new Headers([['a', 'b'],
                                                           ['x', 'y', 'z']]); },
                   'new Headers with a sequence with more than two strings ' +
                   'should throw');
   }, 'Headers');
 
 done();