compositor_unittests target is unimplmented on Mac

ui/gfx/gl/gl.gyp

 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'targets': [
@@ skipping 23 matching lines @@
           '<(gl_binding_output_dir)',
         ],
       },
      'sources': [
         'gl_bindings.h',
         'gl_bindings_skia_in_process.cc',
         'gl_bindings_skia_in_process.h',
         'gl_context.cc',
         'gl_context.h',
         'gl_context_linux.cc',
-        'gl_context_mac.cc',
+        'gl_context_mac.mm',
         'gl_context_osmesa.cc',
         'gl_context_osmesa.h',
         'gl_context_stub.cc',
         'gl_context_stub.h',
         'gl_context_win.cc',
         'gl_export.h',
         'gl_implementation.cc',
         'gl_implementation.h',
         'gl_implementation_linux.cc',
         'gl_implementation_mac.cc',
@@ skipping 110 matching lines @@
             'gl_context_cgl.h',
             'gl_surface_cgl.cc',
             'gl_surface_cgl.h',
           ],
           'link_settings': {
             'libraries': [
               '$(SDKROOT)/System/Library/Frameworks/OpenGL.framework',
             ],
           },
         }],
+        ['OS=="mac" and use_webkit_compositor == 1', {
+          'sources': [
+            'gl_context_nsview.mm',
+            'gl_context_nsview.h',
+            'gl_surface_nsview.mm',
+            'gl_surface_nsview.h',
+          ],
+        }],
       ],
     },
   ],
 }