Bug 1637335 - Make MozEditableElement.editor flush frames if needed. r=smaug

URLBarInput relies on it being non-null when created, but that may not
happen if we construct frames lazily without this patch.

Differential Revision: https://phabricator.services.mozilla.com/D74891

dom/html/HTMLTextAreaElement.cpp

@@ -214,6 +214,13 @@ bool HTMLTextAreaElement::ValueEquals(const nsAString& aValue) const {
   return mState->ValueEquals(aValue);
 }
 
+nsIEditor* HTMLTextAreaElement::GetEditorForBindings() {
+  if (!GetPrimaryFrame()) {
+    GetPrimaryFrame(FlushType::Frames);
+  }
+  return GetTextEditor();
+}
+
 TextEditor* HTMLTextAreaElement::GetTextEditor() {
   MOZ_ASSERT(mState);
   return mState->GetTextEditor();