This was working before somehow… but according to the CodeMirror docs,
it never syncs with the textarea automatically, so regardless this is
a necessary safeguard at least.
Repro steps:
1) Have CodeMirror turned on
2) Open a page for editing
3) Add content
4) Toggle CodeMirror off
With this patch, it should now always be synced.
Also add selenium test to ensure this doesn't break again.
Bug: T317243
Change-Id: Ie44e62fe5838bf32f40c6a3595ec3f541380cfe1