Factor out a BaseContext from JsContext.

gyp/v8.gyp

 # GYP file to build a V8 sample.
 {
   'targets': [
     {
       'target_name': 'SkV8Example',
       'type': 'executable',
       'mac_bundle' : 1,
       'include_dirs' : [
         '../tools/flags',
         '../third_party/externals/v8/include',
         ],
        'sources': [
          '../experimental/SkV8Example/SkV8Example.cpp',
          '../experimental/SkV8Example/SkV8Example.h',
          '../experimental/SkV8Example/Global.cpp',
          '../experimental/SkV8Example/Global.h',
          '../experimental/SkV8Example/Path2D.cpp',
          '../experimental/SkV8Example/Path2D.h',
+         '../experimental/SkV8Example/BaseContext.cpp',
+         '../experimental/SkV8Example/BaseContext.h',
          '../experimental/SkV8Example/JsContext.cpp',
          '../experimental/SkV8Example/JsContext.h',
        ],
        'dependencies': [
          'flags.gyp:flags',
          'skia_lib.gyp:skia_lib',
          'views.gyp:views',
          'xml.gyp:xml',
        ],
        'link_settings': {
@@ skipping 53 matching lines @@
         },
         'mac_bundle_resources' : [
           '../experimental/SkiaExamples/SkiaExamples.xib'
           ],
         }
       ],
      ],
     }
   ],
 }