`HTMLEditor` initializes selection ancestor limit when it receives `focus`
event. If `Document.execCommand()` is called immediately after an
ancestor of active editing host becomes new editing host,
`HTMLEditor::GetActiveEditingHost()` returns the new one, but selection
ancestor limit is still the previous one. This mismatch causes a lot of
bugs. Therefore, this patch makes `nsGenericHTMLElement` notifies `HTMLEditor`
of an element becoming `contenteditable`, and makes `HTMLEditor` update
selection ancestor limit only when the new editing host is ancestor of
old selection ancestor limit.
Differential Revision: https://phabricator.services.mozilla.com/D32823
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extra : moz-landing-system : lando
d9e3ea7e57