Replace WmWindowObserver with aura::WindowObserver.

ash/common/wm/window_positioning_utils.cc

Index: ash/common/wm/window_positioning_utils.cc
diff --git a/ash/common/wm/window_positioning_utils.cc b/ash/common/wm/window_positioning_utils.cc
index aac2949383505fd7346dd8621de1138098bb3906..075090e397681eff224815a6394474c03ea5462f 100644
--- a/ash/common/wm/window_positioning_utils.cc
+++ b/ash/common/wm/window_positioning_utils.cc
@@ -13,8 +13,8 @@
 #include "ash/common/wm_lookup.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
-#include "ash/common/wm_window_tracker.h"
 #include "ash/root_window_controller.h"
+#include "ui/aura/window_tracker.h"
 #include "ui/display/display.h"
 #include "ui/display/types/display_constants.h"
 #include "ui/gfx/geometry/rect.h"
@@ -157,11 +157,11 @@ void SetBoundsInScreen(WmWindow* window,
       WmWindow* focused = WmShell::Get()->GetFocusedWindow();
       WmWindow* active = WmShell::Get()->GetActiveWindow();
 
-      WmWindowTracker tracker;
+      aura::WindowTracker tracker;
       if (focused)
-        tracker.Add(focused);
+        tracker.Add(focused->aura_window());
       if (active && focused != active)
-        tracker.Add(active);
+        tracker.Add(active->aura_window());
 
       gfx::Point origin = bounds_in_screen.origin();
       const gfx::Point display_origin = display.bounds().origin();
@@ -177,11 +177,11 @@ void SetBoundsInScreen(WmWindow* window,
       MoveAllTransientChildrenToNewRoot(display, window);
 
       // Restore focused/active window.
-      if (tracker.Contains(focused)) {
+      if (focused && tracker.Contains(focused->aura_window())) {
         focused->SetFocused();
         WmShell::Get()->set_root_window_for_new_windows(
             focused->GetRootWindow());
-      } else if (tracker.Contains(active)) {
+      } else if (active && tracker.Contains(active->aura_window())) {
         active->Activate();
       }
       // TODO(oshima): We should not have to update the bounds again