Content settings extension API: Implement ContentSetting.getResourceIdentifiers.

chrome/test/data/extensions/api_test/content_settings/getresourceidentifiers/test.html

+<script>
+// Content settings API test
+// Run with browser_tests
+//     --gtest_filter=ExtensionApiTest.ContentSettingsGetResourceIdentifiers
+
+Object.prototype.forEach = function(f) {
+  for (key in this) {
+    if (this.hasOwnProperty(key))
+      f(key, this[key]);
+  }
+}
+
+var cs = chrome.experimental.contentSettings;
+chrome.test.runTests([
+  function getResourceIdentifiers() {
+    var contentTypes = {
+      "cookies": undefined,
+      "images": undefined,
+      "javascript": undefined,
+      "plugins": [
+        {
+          "description": "Foo",
+          "id": "foo",
+        },
+        {
+          "description": "Bar Plugin",
+          "id": "bar.plugin",
+        },
+      ],
+      "popups": undefined,
+      "notifications": undefined
+    };
+    contentTypes.forEach(function(type, identifiers) {
+      cs[type].getResourceIdentifiers(chrome.test.callbackPass(function(value) {
+        chrome.test.assertEq(identifiers, value);
+      }));
+    });
+  },
+]);
+</script>