Cleanup GrContextFactory and make it's subclasses private

gyp/gputest.gyp

 {
   'targets': [
     {
       'target_name': 'skgputest',
       'product_name': 'skia_skgputest',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
       'include_dirs': [
         '../include/gpu',
         '../include/utils',
         '../src/core',
         '../src/gpu',
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../src/gpu',
         ],
       },
       'sources': [
+        '<(skia_src_path)/gpu/GrContextFactory.cpp',
+        '<(skia_src_path)/gpu/GrContextFactory.h',
         '<(skia_src_path)/gpu/GrTest.cpp',
         '<(skia_src_path)/gpu/GrTest.h',
       ],
     },
   ],
 }