Change the visual appearance of panel on GTK per new UI design.

chrome/browser/ui/panels/panel_constants.h

 // Copyright (c) 2012 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 CHROME_BROWSER_UI_PANELS_PANEL_CONSTANTS_H_
 #define CHROME_BROWSER_UI_PANELS_PANEL_CONSTANTS_H_
 #pragma once
 
 namespace panel {
 
 // Absolute minimum width and height for panels, including non-client area.
 // Should only be big enough to accomodate a close button on the reasonably
 // recognisable titlebar.
 // These numbers are semi-arbitrary.
 // Motivation for 'width' is to make main buttons on the titlebar functional.
 // Motivation for height is to allow autosized tightly-wrapped panel with a
 // single line of text - so the height is set to be likely less then a titlebar,
 // to make sure even small content is tightly wrapped.
 const int kPanelMinWidth = 80;
 const int kPanelMinHeight = 20;
 
 // The panel can be minimized to 4-pixel lines.
 static const int kMinimizedPanelHeight = 4;
 
 // The height in pixels of the titlebar.
 static const int kTitlebarHeight = 32;
 
+// Different types of buttons that can be shown on panel's titlebar.
+enum TitlebarButtonType {
+  CLOSE_BUTTON,
+  MINIMIZE_BUTTON,
+  RESTORE_BUTTON
+};
+
 // Different platforms use different modifier keys to change the behavior
 // of a mouse click. This enum captures the meaning of the modifier rather
 // than the actual modifier key to generalize across platforms.
 enum ClickModifier {
   NO_MODIFIER,
   APPLY_TO_ALL,  // Apply the click behavior to all panels in the strip.
 };
 
 // Edge at which a panel is being resized using the mouse.
 enum ResizingSides {
@@ skipping 11 matching lines @@
 // Ways a panel can be resized.
 enum Resizability {
   NOT_RESIZABLE,
   RESIZABLE_ALL_SIDES,
   RESIZABLE_ALL_SIDES_EXCEPT_BOTTOM
 };
 
 }  // namespace panel
 
 #endif  // CHROME_BROWSER_UI_PANELS_PANEL_CONSTANTS_H_