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

chrome/browser/ui/panels/panel_constants.h

Index: chrome/browser/ui/panels/panel_constants.h
diff --git a/chrome/browser/ui/panels/panel_constants.h b/chrome/browser/ui/panels/panel_constants.h
index f51b7b5ba00e00470c440cc03e049dec2f66db8b..d805c69fe8bdf207b86920608f8d7db663478a18 100644
--- a/chrome/browser/ui/panels/panel_constants.h
+++ b/chrome/browser/ui/panels/panel_constants.h
@@ -25,6 +25,13 @@ 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.