[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..b6246f90de436f05e955ea4ad52b094c1f7c0287 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 +100,22 @@ void MediaRouterDialogDelegate::GetDialogSize(gfx::Size* size) const {
 
 }  // namespace
 
+// static
+MediaRouterDialogController*
+MediaRouterDialogController::GetOrCreateForWebContents(
+    WebContents* web_contents) {
+  DCHECK(web_contents);
+  MediaRouterDialogController* controller =
+      media_router::MediaRouterDialogController::FromWebContents(web_contents);

[Peter Kasting, 2015/06/04 00:17:53]

Nit: media_router:: namespace qualifier not necessary (and you don't use it
below)

[imcheng (use chromium acct), 2015/06/04 01:04:20]

Done.

+  if (controller)
+    return controller;
+
+  MediaRouterDialogController::CreateForWebContents(web_contents);
+  controller = MediaRouterDialogController::FromWebContents(web_contents);
+  DCHECK(controller);
+  return controller;

[Peter Kasting, 2015/06/04 00:17:53]

Nit: This function is more verbose than it needs to be, because
CreateForWebContents() will automatically no-op if the object exists.  So just
something like:

{
  // This call does nothing if the controller already exists.
  MediaRouterDialogController::CreateForWebContents(web_contents);
  return MediaRouterDialogController::FromWebContents(web_contents);
}

[imcheng (use chromium acct), 2015/06/04 01:04:20]

My reasoning was that although CreateForWebContents is no-op when called a
second time, it still has to check for the existence for the
MediaRouterDialogController in order for it to no-op. It's also an extra
function call. But I guess it does not make a big difference in terms of
performance, so I took your suggestion.

+}
+
 class MediaRouterDialogController::DialogWebContentsObserver
     : public content::WebContentsObserver {
  public: