Converts MruWindowTracker to work with common types

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 02f4f6f8819f141c44ca9c7a4d55b425853e5c88..cfe005e1d8fc9550daf310d26abee4993b391957 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -1048,8 +1048,7 @@ void Shell::Init(const ShellInitParams& init_params) {
 
   magnification_controller_.reset(
       MagnificationController::CreateInstance());
-  mru_window_tracker_.reset(new MruWindowTracker(activation_client_,
-                                                 focus_rules));
+  mru_window_tracker_.reset(new MruWindowTracker);
 
   partial_magnification_controller_.reset(
       new PartialMagnificationController());