Bug 1531693 - Relax the assertion for another editor handling the composition to take account of cycle collection. r=masayuki

Differential Revision: https://phabricator.services.mozilla.com/D24529

--HG--
extra : moz-landing-system : lando

dom/events/TextComposition.cpp

@@ -651,7 +651,7 @@ void TextComposition::EndHandlingComposition(EditorBase* aEditorBase) {
 
 #ifdef DEBUG
   RefPtr<EditorBase> editorBase = GetEditorBase();
-  MOZ_ASSERT(editorBase == aEditorBase,
+  MOZ_ASSERT(!editorBase || editorBase == aEditorBase,
              "Another editor handled the composition?");
 #endif  // #ifdef DEBUG
   mEditorBaseWeak = nullptr;