Use active window if on current workspace for fullscreen mode.

ash/wm/mru_window_tracker.h

Index: ash/wm/mru_window_tracker.h
diff --git a/ash/wm/mru_window_tracker.h b/ash/wm/mru_window_tracker.h
index 3f8740d47c0873b7c304b79b260e6502e705b9ae..430af1148119890a0fb9ee52de1b672479c8bea5 100644
--- a/ash/wm/mru_window_tracker.h
+++ b/ash/wm/mru_window_tracker.h
@@ -61,6 +61,10 @@ class ASH_EXPORT MruWindowTracker
   // windows to the front of the MRU window list.
   void SetIgnoreActivations(bool ignore);
 
+  // Returns the most recently used window in |root_window| or NULL if none
+  // found.
+  const aura::Window* GetMruWindowInRoot(const aura::Window* root_window);
+
  private:
   // Updates the mru_windows_ list to insert/move |active_window| at/to the
   // front.