Only for solving the order of `AutoEditInitRulesTrigger` destruction when
`HTMLEditor::Init()` calls `TextEditor::Init()`, `TextEditor` has unnecessary
counter and the initialization code is made harder to understand.
Therefore, this patch makes `TextEditor::Init()` and `HTMLEditor::Init()`
directly call `InitEditorContentAndSelection()` at appropriate time.
Additionally, `HTMLEditor::Init()` can call `EditorBase::Init()` instead of
`TextEditor::Init()` since `TextEditor::Init()` does nothing for `HTMLEditor`
actually.
Differential Revision: https://phabricator.services.mozilla.com/D45793
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extra : moz-landing-system : lando
9973ff7319