Adds a gfx_export.h to ui/gfx and converts all code in ui/gfx to

ui/gfx/selection_model.h

 // Copyright (c) 2012 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_SELECTION_MODEL_H_
 #define UI_GFX_SELECTION_MODEL_H_
 
 #include <string>
 
 #include "ui/base/range/range.h"
-#include "ui/base/ui_export.h"
+#include "ui/gfx/gfx_export.h"
 
 namespace gfx {
 
 // VisualCursorDirection and LogicalCursorDirection represent directions of
 // motion of the cursor in BiDi text. The combinations that make sense are:
 //
 //  base::i18n::TextDirection  VisualCursorDirection  LogicalCursorDirection
 //       LEFT_TO_RIGHT             CURSOR_LEFT           CURSOR_BACKWARD
 //       LEFT_TO_RIGHT             CURSOR_RIGHT          CURSOR_FORWARD
 //       RIGHT_TO_LEFT             CURSOR_RIGHT          CURSOR_BACKWARD
@@ skipping 82 matching lines @@
   //          3                  abc|FED                     abcFED|
   //          4                  abcFE|D                     abcFE|D
   //          5                  abcF|ED                     abcF|ED
   //          6                  abc|FED                     abcFED|
   LogicalCursorDirection caret_affinity_;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_SELECTION_MODEL_H_