exo: Fix dragging edge cases

components/exo/wm_helper.h

Index: components/exo/wm_helper.h
diff --git a/components/exo/wm_helper.h b/components/exo/wm_helper.h
index 69798803792ddb82bd4eafa675ee77b7b48e2210..eba40c8df456696ade8b017c93a35833dc9f65c8 100644
--- a/components/exo/wm_helper.h
+++ b/components/exo/wm_helper.h
@@ -53,13 +53,16 @@ class WMHelper {
     virtual ~CursorObserver() {}
   };
 
-  class MaximizeModeObserver {
+  class ShellObserver {
    public:
-    virtual void OnMaximizeModeStarted() = 0;
-    virtual void OnMaximizeModeEnded() = 0;
+    virtual void OnMaximizeModeStarted() {}
+    virtual void OnMaximizeModeEnded() {}
+
+    virtual void OnOverviewModeStarted() {}
+    virtual void OnOverviewModeEnded() {}
 
    protected:
-    virtual ~MaximizeModeObserver() {}
+    virtual ~ShellObserver() {}
   };
 
   virtual ~WMHelper();
@@ -73,8 +76,8 @@ class WMHelper {
   void RemoveFocusObserver(FocusObserver* observer);
   void AddCursorObserver(CursorObserver* observer);
   void RemoveCursorObserver(CursorObserver* observer);
-  void AddMaximizeModeObserver(MaximizeModeObserver* observer);
-  void RemoveMaximizeModeObserver(MaximizeModeObserver* observer);
+  void AddShellObserver(ShellObserver* observer);
+  void RemoveShellObserver(ShellObserver* observer);
 
   virtual const display::ManagedDisplayInfo GetDisplayInfo(
       int64_t display_id) const = 0;
@@ -100,12 +103,14 @@ class WMHelper {
   void NotifyCursorSetChanged(ui::CursorSetType cursor_set);
   void NotifyMaximizeModeStarted();
   void NotifyMaximizeModeEnded();
+  void NotifyOverviewModeStarted();
+  void NotifyOverviewModeEnded();
 
  private:
   base::ObserverList<ActivationObserver> activation_observers_;
   base::ObserverList<FocusObserver> focus_observers_;
   base::ObserverList<CursorObserver> cursor_observers_;
-  base::ObserverList<MaximizeModeObserver> maximize_mode_observers_;
+  base::ObserverList<ShellObserver> shell_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WMHelper);
 };