athena: Add a minimal executable for just the UI.

athena/main/athena_main.gyp

Index: athena/main/athena_main.gyp
diff --git a/athena/main/athena_main.gyp b/athena/main/athena_main.gyp
index 480f40c98e3a2db5e5d86918749c35d41a964266..c21fa1116d33c35a31135cbe447bfbdcf52e83f2 100644
--- a/athena/main/athena_main.gyp
+++ b/athena/main/athena_main.gyp
@@ -28,5 +28,23 @@
         'placeholder.h',
       ],
     },
+    {
+      'target_name': 'athena_shell',
+      'type': 'executable',
+      'dependencies': [
+        '../../base/base.gyp:base',
+        '../../base/base.gyp:base_i18n',
+        '../../skia/skia.gyp:skia',
+        '../../ui/accessibility/accessibility.gyp:ax_gen',
+        '../../ui/aura/aura.gyp:aura',
+        '../../ui/compositor/compositor.gyp:compositor_test_support',
+        '../../ui/gfx/gfx.gyp:gfx',
+        '../athena.gyp:athena_lib',
+        '../athena.gyp:athena_test_support',
+      ],
+      'sources': [
+        'athena_shell.cc',
+      ],
+    }
   ],  # targets
 }