Enable overview mode by default.

ash/wm/overview/window_selector_controller.cc

Index: ash/wm/overview/window_selector_controller.cc
diff --git a/ash/wm/overview/window_selector_controller.cc b/ash/wm/overview/window_selector_controller.cc
index 84c83d20699bf03cb275916c02a83a29a8ba6431..f9839dcb43a7669f5ad66334d5821abc65cf33ef 100644
--- a/ash/wm/overview/window_selector_controller.cc
+++ b/ash/wm/overview/window_selector_controller.cc
@@ -50,6 +50,10 @@ void WindowSelectorController::HandleCycleWindow(
   if (!IsSelecting()) {
     std::vector<aura::Window*> windows = ash::Shell::GetInstance()->
         mru_window_tracker()->BuildMruWindowList();
+    // Don't cycle with no windows.
+    if (windows.empty())
+      return;
+
     // Removing focus will hide popup windows like the omnibar or open menus.
     window_selector_.reset(
         new WindowSelector(windows, WindowSelector::CYCLE, this));