Implement SkAshmemDiscardableMemory

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:*',
@@ skipping 135 matching lines @@
             '../src/ports/SkFontMgr_default_dw.cpp',
             '../src/ports/SkOSFile_win.cpp',
             '../src/ports/SkThread_win.cpp',
             '../src/ports/SkTime_win.cpp',
             '../src/ports/SkTLS_win.cpp',
           ],
         }],
         [ 'skia_os == "android"', {
           'sources!': [
             '../src/ports/SkDebug_stdio.cpp',
+            '../src/ports/SkDiscardableMemory_none.cpp',
             '../src/ports/SkPurgeableMemoryBlock_none.cpp',
           ],
           'sources': [
             '../src/ports/SkDebug_android.cpp',
+            '../src/ports/SkDiscardableMemory_ashmem.cpp',
             '../src/ports/SkFontConfigInterface_android.cpp',
             '../src/ports/SkFontConfigParser_android.cpp',
             '../src/ports/SkFontHost_fontconfig.cpp',
             '../src/ports/SkPurgeableMemoryBlock_android.cpp',
           ],
           'dependencies': [
              'android_deps.gyp:expat',
           ],
         }],
       ],
       'direct_dependent_settings': {
         'include_dirs': [
           '../include/ports',
         ],
       },
     },
   ],
 }