Add options and a browser-action popup.

go-back-with-backspace/is_editable.js

+// Copyright 2016 Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 // Determine whether focus is in an editable text field.
 function isEditable(target) {
   // Elements may be explicitly marked as editable.
   if (target.isContentEditable)
     return true;
 
   // Entire documents may be editable.
   if (target.ownerDocument.designMode.toLowerCase() === 'on')
     return true;
 
@@ skipping 26 matching lines @@
       if (userModify === 'read-write' ||
           userModify === 'write-only' ||
           userModify === 'read-write-plaintext-only') {
         return true;
       }
     }
     target = target.parentNode || null;
   }
   return false;
 }