Create abstract common base class for impl scrollbars.

cc/cc.gyp

 # 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.
 
 {
   'variables': {
     'cc_source_files': [
       'active_animation.cc',
       'active_animation.h',
       'animation_curve.cc',
@@ skipping 194 matching lines @@
       'scrollbar_animation_controller_linear_fade.cc',
       'scrollbar_animation_controller_linear_fade.h',
       'scrollbar_geometry_fixed_thumb.cc',
       'scrollbar_geometry_fixed_thumb.h',
       'scrollbar_geometry_stub.cc',
       'scrollbar_geometry_stub.h',
       'scrollbar_layer.cc',
       'scrollbar_layer.h',
       'scrollbar_layer_impl.cc',
       'scrollbar_layer_impl.h',
+      'scrollbar_layer_impl_base.h',
       'shader.cc',
       'shader.h',
       'shared_quad_state.cc',
       'shared_quad_state.h',
       'single_thread_proxy.cc',
       'single_thread_proxy.h',
       'skpicture_content_layer_updater.cc',
       'skpicture_content_layer_updater.h',
       'software_output_device.h',
       'software_renderer.cc',
@@ skipping 82 matching lines @@
       ],
       'all_dependent_settings': {
         'include_dirs': [
           # Needed for <public/WebTransformationMatrix.h> in layer.h
           '<(webkit_src_dir)/Source/Platform/chromium',
         ],
       }
     },
   ],
 }