Cache a screen compatible DC to avoid re-creating it excessively.

ui/gfx/screen.h

Index: ui/gfx/screen.h
===================================================================
--- ui/gfx/screen.h	(revision 120215)
+++ ui/gfx/screen.h	(working copy)
@@ -65,6 +65,16 @@
   // number of distinct, usable displays.
   static int GetNumMonitors();
 
+#if defined(OS_WIN)

[sky, 2012/02/03 18:34:06]

I don't like defining this here. Move it into it's own class/namespace.

+  // Returns a device context compatible with the screen. The context can be
+  // used as a temporary context on the UI thread.
+  static HDC GetCompatibleDC();
+
+  // Should be called on WM_DISPLAYCHANGE to release the cached compatible
+  // device context, if any.
+  static void ReleaseCompatibleDC();
+#endif
+
  protected:
   virtual gfx::Point GetCursorScreenPointImpl() = 0;
   virtual gfx::Rect GetMonitorWorkAreaNearestWindowImpl(
@@ -83,6 +93,11 @@
   // The singleton screen instance. Only used on aura.
   static Screen* instance_;
 #endif
+
+#if defined(OS_WIN)
+  // Cached device context compatible with the screen.
+  static HDC compatible_dc_;
+#endif
 };
 
 }  // namespace gfx