[Media Router WebUI] Use I18nBehavior to retrieve strings.

chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js

Index: chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js
diff --git a/chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js b/chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js
index 6d61a666559f5ba25eddee5914702b9923811af8..93b74981c1f0c4a28270cae795f48475d72cbaab 100644
--- a/chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js
+++ b/chrome/test/data/webui/media_router/media_router_container_cast_mode_list_tests.js
@@ -156,7 +156,7 @@ cr.define('media_router_container_cast_mode_list', function() {
       // Tests the header text. Choosing a cast mode updates the header text.
       test('header text with no default cast modes', function(done) {
         assertEquals(loadTimeData.getString('selectCastModeHeaderText'),
-            container.i18n_.selectCastModeHeaderText);
+            container.i18n('selectCastModeHeaderText'));
 
         // The container is currently in auto cast mode, since we have not
         // picked a cast mode explicitly, and the sinks is not compatible