Fix |RunMicrotasks()| leaking reference to the last context being run on.

src/object-observe.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 416 matching lines @@
       // Drop all splice records on the floor for access-checked objects
       (changeRecord.type == 'splice' ||
        !%IsAccessAllowedForObserver(
            callback, changeRecord.object, changeRecord.name))) {
     return;
   }
 
   var callbackInfo = CallbackInfoNormalize(callback);
   if (IS_NULL(GetPendingObservers())) {
     SetPendingObservers(nullProtoObject())
-    GetMicrotaskQueue().push(ObserveMicrotaskRunner);
-    %SetMicrotaskPending(true);
+    EnqueueMicrotask(ObserveMicrotaskRunner);
   }
   GetPendingObservers()[callbackInfo.priority] = callback;
   callbackInfo.push(changeRecord);
 }
 
 function ObjectInfoEnqueueExternalChangeRecord(objectInfo, changeRecord, type) {
   if (!ObjectInfoHasActiveObservers(objectInfo))
     return;
 
   var hasType = !IS_UNDEFINED(type);
@@ skipping 191 matching lines @@
     "observe", ArrayObserve,
     "unobserve", ArrayUnobserve
   ));
   InstallFunctions(notifierPrototype, DONT_ENUM, $Array(
     "notify", ObjectNotifierNotify,
     "performChange", ObjectNotifierPerformChange
   ));
 }
 
 SetupObjectObserve();