Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

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': {
     'chromium_code': 1,
     'cc_source_files': [
       'animation/animation.cc',
       'animation/animation.h',
@@ skipping 63 matching lines @@
       'debug/traced_picture.cc',
       'debug/traced_picture.h',
       'debug/traced_value.cc',
       'debug/traced_value.h',
       'input/input_handler.h',
       'input/page_scale_animation.cc',
       'input/page_scale_animation.h',
       'input/top_controls_manager.cc',
       'input/top_controls_manager.h',
       'input/top_controls_manager_client.h',
+      'input/viewport_scrollbar.cc',
+      'input/viewport_scrollbar.h',
       'layers/append_quads_data.h',
       'layers/compositing_reasons.h',
       'layers/content_layer.cc',
       'layers/content_layer.h',
       'layers/content_layer_client.h',
       'layers/contents_scaling_layer.cc',
       'layers/contents_scaling_layer.h',
       'layers/delegated_renderer_layer.cc',
       'layers/delegated_renderer_layer.h',
       'layers/delegated_renderer_layer_client.h',
@@ skipping 274 matching lines @@
         'CC_IMPLEMENTATION=1',
       ],
       'sources': [
         '<@(cc_source_files)',
       ],
       # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],
     },
   ],
 }