Linux UI: Dynamically respond to changes in the scale factor

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
index c0e1763449def134e9b90d9516af60b578f56c9a..0b5e06ca3cbad1ddc9712cd37861a74b6f388e9d 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
@@ -18,6 +18,7 @@
 #include "ui/aura/scoped_window_targeter.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/base/cursor/cursor_loader_x11.h"
+#include "ui/display/display_observer.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/insets.h"
 #include "ui/gfx/geometry/rect.h"
@@ -44,7 +45,8 @@ class X11DesktopWindowMoveClient;
 class VIEWS_EXPORT DesktopWindowTreeHostX11
     : public DesktopWindowTreeHost,
       public aura::WindowTreeHost,
-      public ui::PlatformEventDispatcher {
+      public ui::PlatformEventDispatcher,
+      public display::DisplayObserver {
  public:
   DesktopWindowTreeHostX11(
       internal::NativeWidgetDelegate* native_widget_delegate,
@@ -169,6 +171,12 @@ class VIEWS_EXPORT DesktopWindowTreeHostX11
       const gfx::Point& location_in_pixels) override;
   void OnCursorVisibilityChangedNative(bool show) override;
 
+  // Overridden from display::DisplayObserver:
+  void OnDisplayAdded(const display::Display& new_display) override;
+  void OnDisplayRemoved(const display::Display& old_display) override;
+  void OnDisplayMetricsChanged(const display::Display& display,
+                               uint32_t changed_metrics) override;
+
  private:
   friend class DesktopWindowTreeHostX11HighDPITest;
   // Initializes our X11 surface to draw on. This method performs all