linux_aura: Use system configuration for middle clicking the titlebar.

ui/views/linux_ui/linux_ui.h

Index: ui/views/linux_ui/linux_ui.h
diff --git a/ui/views/linux_ui/linux_ui.h b/ui/views/linux_ui/linux_ui.h
index 563a518bfe55b6550468e279c4ca3fcce83a88de..cadc7b72632b51f0806acff2cb51c3e7621dd637 100644
--- a/ui/views/linux_ui/linux_ui.h
+++ b/ui/views/linux_ui/linux_ui.h
@@ -46,6 +46,15 @@ class VIEWS_EXPORT LinuxUI : public ui::LinuxInputMethodContextFactory,
                              public ui::LinuxShellDialog,
                              public ui::TextEditKeyBindingsDelegateAuraLinux {
  public:
+  // Describes the window management actions that could be taken in response to
+  // a middle click in the non client area.
+  enum NonClientMiddleClickAction {
+    MIDDLE_CLICK_ACTION_NONE,
+    MIDDLE_CLICK_ACTION_LOWER,
+    MIDDLE_CLICK_ACTION_MINIMIZE,
+    MIDDLE_CLICK_ACTION_TOGGLE_MAXIMIZE
+  };
+
   virtual ~LinuxUI() {}
 
   // Sets the dynamically loaded singleton that draws the desktop native UI.
@@ -127,6 +136,10 @@ class VIEWS_EXPORT LinuxUI : public ui::LinuxInputMethodContextFactory,
   // Determines whether the user's window manager is Unity.
   virtual bool UnityIsRunning() = 0;
 
+  // What action we should take when the user middle clicks on non-client
+  // area. The default is lowering the window.
+  virtual NonClientMiddleClickAction GetNonClientMiddleClickAction() = 0;
+
   // Notifies the window manager that start up has completed.
   // Normally Chromium opens a new window on startup and GTK does this
   // automatically. In case Chromium does not open a new window on startup,