Bug 1435295 - Don't attempt to mark object group unknown unnecessarily when merging compartments r=jandem

js/src/jit-test/tests/gc/bug-1435295.js

@@ -0,0 +1,11 @@
+if (helperThreadCount() === 0)
+    quit();
+if (!('oomTest' in this))
+    quit();
+
+oomTest(new Function(`function execOffThread(source) {
+    offThreadCompileModule(source);
+    return finishOffThreadModule();
+}
+b = execOffThread("[1, 2, 3]")
+`));

js/src/jsgc.cpp

@@ -7934,7 +7934,7 @@ GCRuntime::mergeCompartments(JSCompartment* source, JSCompartment* target)
                 JSObject* targetProto = global->getPrototypeForOffThreadPlaceholder(obj);
                 MOZ_ASSERT(targetProto->isDelegate());
                 group->setProtoUnchecked(TaggedProto(targetProto));
-                if (targetProto->isNewGroupUnknown())
+                if (targetProto->isNewGroupUnknown() && !group->unknownProperties())
                     group->markUnknown(cx);
             }
         }