The purpose of this patch is to prevent users to make changes to
read-only editors.
This patch forces `IsCommandEnabled()` to return false for read-only
editors for "standard" editor commands that could have an associated
icon or menu entry and are therefore user-accessible.
This patch does not modify/test any paste commands or any of the
advanced internal editor commands.
Differential Revision: https://phabricator.services.mozilla.com/D203754
89637d1080