Bug 1850293 - Make editable states not intrinsic. r=smaug,masayuki

This one is tricky because form controls, though I think I got it right...

This fixes a pre-existing bug where we're not following the spec for readonly
inside content-editable.

I filed this as bug 1850390 but other browsers match the spec so add a test and
fix it while at it. This allows cheaper checks for readonlyness in a couple
places.

Differential Revision: https://phabricator.services.mozilla.com/D186896

dom/html/TextControlState.cpp

@@ -1883,11 +1883,8 @@ nsresult TextControlState::PrepareEditor(const nsAString* aValue) {
 
   editorFlags = newTextEditor->Flags();
 
-  // Check if the readonly attribute is set.
-  //
-  // TODO: Should probably call IsDisabled(), as it is cheaper.
-  if (mTextCtrlElement->HasAttr(nsGkAtoms::readonly) ||
-      mTextCtrlElement->HasAttr(nsGkAtoms::disabled)) {
+  // Check if the readonly/disabled attributes are set.
+  if (mTextCtrlElement->IsDisabledOrReadOnly()) {
     editorFlags |= nsIEditor::eEditorReadonlyMask;
   }