Put common constants in atom_constants

atom/common/atom_constants.h

@@ -0,0 +1,15 @@
+// Copyright (c) 2015 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_COMMON_ATOM_CONSTANTS_H_
+#define ATOM_COMMON_ATOM_CONSTANTS_H_
+
+namespace atom {
+
+// Header to ignore CORS.
+extern const char* kCORSHeader;
+
+}  // namespace atom
+
+#endif  // ATOM_COMMON_ATOM_CONSTANTS_H_