[Media Router] Implement MediaRouterAction + MediaRouterUI tests.

chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc

Index: chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc
diff --git a/chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc b/chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc
index 1265f53dd411b25d3d72ea2e29ef12ad56b4427a..546eaee3634d1a927526aff090da75097b93b458 100644
--- a/chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc
+++ b/chrome/browser/ui/webui/media_router/media_router_dialog_controller.cc
@@ -100,6 +100,16 @@ void MediaRouterDialogDelegate::GetDialogSize(gfx::Size* size) const {
 
 }  // namespace
 
+// static
+MediaRouterDialogController*
+MediaRouterDialogController::GetOrCreateForWebContents(
+    WebContents* web_contents) {
+  DCHECK(web_contents);
+  // This call does nothing if the controller already exists.
+  MediaRouterDialogController::CreateForWebContents(web_contents);
+  return MediaRouterDialogController::FromWebContents(web_contents);
+}
+
 class MediaRouterDialogController::DialogWebContentsObserver
     : public content::WebContentsObserver {
  public: