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

src/promise.js

Index: src/promise.js
diff --git a/src/promise.js b/src/promise.js
index 27890a7626b37d12af837e73627617d6a94da4f2..e02b494e17b27e6f165ab0dc4b36f72f956a2da6 100644
--- a/src/promise.js
+++ b/src/promise.js
@@ -173,13 +173,11 @@ function PromiseCatch(onReject) {
 }
 
 function PromiseEnqueue(value, tasks) {
-  GetMicrotaskQueue().push(function() {
+  EnqueueMicrotask(function() {
     for (var i = 0; i < tasks.length; i += 2) {
       PromiseHandle(value, tasks[i], tasks[i + 1])
     }
   });
-
-  %SetMicrotaskPending(true);
 }
 
 function PromiseHandle(value, handler, deferred) {