views: Factor out SingleSplitView Observer into SingleSplitViewListener.

views/controls/single_split_view_listener.h

Index: views/controls/single_split_view_listener.h
diff --git a/views/controls/single_split_view_listener.h b/views/controls/single_split_view_listener.h
new file mode 100644
index 0000000000000000000000000000000000000000..6d38aad67327c8f6c5731806708c25a4cb8c2531
--- /dev/null
+++ b/views/controls/single_split_view_listener.h
@@ -0,0 +1,29 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef VIEWS_CONTROLS_SINGLE_SPLIT_VIEW_LISTENER_H_
+#define VIEWS_CONTROLS_SINGLE_SPLIT_VIEW_LISTENER_H_
+#pragma once
+
+namespace views {
+
+class SingleSplitView;
+
+// An interface implemented by objects that want to be notified when the
+// splitter moves.
+class SingleSplitViewListener {
+ public:
+  // Invoked when split handle is moved by the user. |sender|'s divider_offset
+  // is already set to the new value, but Layout has not happened yet.
+  // Returns false if the layout has been handled by the listener, returns
+  // true if |sender| should do it by itself.
+  virtual bool SplitHandleMoved(SingleSplitView* sender) = 0;
+
+ protected:
+  virtual ~SingleSplitViewListener() {}
+};
+
+}  // namespace views
+
+#endif  // VIEWS_CONTROLS_SINGLE_SPLIT_VIEW_LISTENER_H_