Change GR_COMPRESS_ALPHA_MASK from compile-time flag to run-time flag

gyp/gpu.gyp

 # GYP for building gpu
 {
   'target_defaults': {
     'conditions': [
       ['skia_os != "win"', {
         'sources/': [ ['exclude', '_win.(h|cpp)$'],
         ],
       }],
       ['skia_os != "mac"', {
         'sources/': [ ['exclude', '_mac.(h|cpp|m|mm)$'],
@@ skipping 56 matching lines @@
         [ 'skia_os == "win"', {
           'defines': [
             'GR_GL_FUNCTION_TYPE=__stdcall',
           ],
         }],
       ],
       'include_dirs': [
         '../include/gpu',
       ],
     },
-    'defines': [
-      'GR_COMPRESS_ALPHA_MASK=0',
-    ],
   },
   'targets': [
     {
       'target_name': 'skgpu',
       'product_name': 'skia_skgpu',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'core.gyp:*',
         'utils.gyp:*',
@@ skipping 132 matching lines @@
             'libraries': [
               '-lGLESv2',
               '-lEGL',
             ],
           },
         }],
       ],
     },
   ],
 }