Enable hidpi on Linux, refactor a bit on Windows to share

ui/gfx/canvas_paint_win.h

 // Copyright (c) 2011 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 UI_GFX_CANVAS_PAINT_WIN_H_
 #define UI_GFX_CANVAS_PAINT_WIN_H_
 
 #include "skia/ext/platform_canvas.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/dpi.h"
 #include "ui/gfx/size.h"
-#include "ui/gfx/win/dpi.h"
 
 namespace gfx {
 
 // A class designed to help with WM_PAINT operations on Windows. It will create
 // the bitmap and canvas with the correct size and transform for the dirty rect.
 // The bitmap will be automatically painted to the screen on destruction.
 //
 // You MUST call isEmpty before painting to determine if anything needs
 // painting. Sometimes the dirty rect can actually be empty, and this makes
 // the bitmap functions we call unhappy. The caller should not paint in this
@@ skipping 45 matching lines @@
   HDC paint_dc_;
   PAINTSTRUCT ps_;
 
   // Disallow copy and assign.
   DISALLOW_COPY_AND_ASSIGN(CanvasSkiaPaint);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_CANVAS_PAINT_WIN_H_