Add support for using expressions as enum values.

mojo/public/bindings/sample/sample_service.mojom

 // Copyright 2013 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.
 
 module sample {
 
 enum BarType {
-  BAR_VERTICAL = 1,
-  BAR_HORIZONTAL = 2,
+  BAR_VERTICAL = 1 << 0,
+  BAR_HORIZONTAL = (1 << 1) + 0,
+  BAR_BOTH = BAR_VERTICAL | BAR_HORIZONTAL,
   BAR_INVALID
 };
 
 struct Bar {
   uint8 alpha @0;
   uint8 beta @1;
   uint8 gamma @2;
   int32 type @3;
 };
 
@@ skipping 21 matching lines @@
   };
   void Frobinate(Foo foo @0, int32 baz @1, handle<message_pipe> port @2) @0;
 };
 
 [Peer=Service]
 interface ServiceClient {
   void DidFrobinate(int32 result @0) @0;
 };
 
 }