Add custom action support

ui/accessibility/ax_enums.idl

Index: ui/accessibility/ax_enums.idl
diff --git a/ui/accessibility/ax_enums.idl b/ui/accessibility/ax_enums.idl
index 751a9b9f18094466fb6468e28e5ed62d8b0cad5b..7eb34fe1cbba7edcb0d6c6a1b292420e112ced25 100644
--- a/ui/accessibility/ax_enums.idl
+++ b/ui/accessibility/ax_enums.idl
@@ -532,7 +532,13 @@
     // For inline text. These int lists must be the same size; they represent
     // the start and end character offset of each word within this text.
     word_starts,
-    word_ends
+    word_ends,
+
+    custom_action_ids

[David Tseng, 2017/05/15 15:20:00]

nit: .. // arc

+  };
+
+  [cpp_enum_prefix_override="ax_attr"] enum AXStringListAttribute {
+    custom_action_descriptions

[David Tseng, 2017/05/15 15:20:00]

You can avoid doing this if you delimit via commas and add this as a string
attribute. You could then avoid adding string attribute support throughout

 Optional though, as I think this might be useful for other attributes in the
future.

[yawano, 2017/05/15 23:18:16]

I think adding string list attribute is more straightforward. An application can
use arbitrary characters for its description including commas. If we use comma
separated list, we need to workaround for those descriptions.

[David Tseng, 2017/05/16 22:18:55]

Fair enough. You'll have to add string list attribute support through.

   };
 
   enum AXMarkerType {