Bug 1683226 - part 1: Make IMEState::Enabled an enum class r=m_kato,geckoview-reviewers

Before deleting `IMEState::Enabled::PLUGIN`, let's make it an enum class
for making the change safer.  Almost all of this change is done by
"replace" of VSCode.

Differential Revision: https://phabricator.services.mozilla.com/D100100

dom/html/HTMLObjectElement.cpp

@@ -339,7 +339,7 @@ bool HTMLObjectElement::IsHTMLFocusable(bool aWithMouse, bool* aIsFocusable,
 
 nsIContent::IMEState HTMLObjectElement::GetDesiredIMEState() {
   if (Type() == eType_Plugin) {
-    return IMEState(IMEState::PLUGIN);
+    return IMEState(IMEEnabled::Plugin);
   }
 
   return nsGenericHTMLFormElement::GetDesiredIMEState();