skia: Added SkDiscardableMemory interface.

gyp/ports.gyp

 # Port-specific Skia library code.
 {
   'targets': [
     {
       'target_name': 'ports',
       'product_name': 'skia_ports',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'core.gyp:*',
       ],
       'include_dirs': [
         '../include/effects',
         '../include/images',
         '../include/ports',
         '../include/utils',
         '../include/utils/win',
         '../include/xml',
         '../src/core',
         '../src/lazy',
         '../src/ports',
         '../src/sfnt',
         '../src/utils',
       ],
       'sources': [
         '../src/ports/SkDebug_nacl.cpp',
         '../src/ports/SkDebug_stdio.cpp',
         '../src/ports/SkDebug_win.cpp',
+        '../src/ports/SkDiscardableMemory_none.cpp',
         '../src/ports/SkFontHost_win.cpp',
         '../src/ports/SkFontHost_win_dw.cpp',
         '../src/ports/SkGlobalInitialization_default.cpp',
         '../src/ports/SkMemory_malloc.cpp',
         '../src/ports/SkOSFile_posix.cpp',
         '../src/ports/SkOSFile_stdio.cpp',
         '../src/ports/SkOSFile_win.cpp',
         '../src/ports/SkPurgeableMemoryBlock_none.cpp',
        #'../src/ports/SkThread_none.cpp',
         '../src/ports/SkThread_pthread.cpp',
@@ skipping 127 matching lines @@
       },
     },
   ],
 }
 
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