Turning on warning as error on Linux

gyp/utils.gyp

 {
   'targets': [
     {
       'target_name': 'utils',
       'product_name': 'skia_utils',
       'type': 'static_library',
       'standalone_static_library': 1,
       'dependencies': [
         'cityhash',
       ],
@@ skipping 240 matching lines @@
               'WarnAsError': 'false',
             },
           },
         }],
         [ 'skia_os == "android"', {
           'cflags!': [
             '-Wall',
             '-Werror',
           ],
         }],
+        ['skia_os in ["linux", "freebsd", "openbsd", "solaris", "nacl"]', {
+          'cflags!': [
+            '-Werror',
+          ],
+        }],
       ],
     },
   ],
 }
 
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