Runtime::RunMicrotask should silent return if no pending microtask work (rather than asserting)

test/mjsunit/fuzz-natives-part3.js

 // Copyright 2011 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 199 matching lines @@
 
   // Only applicable to generators.
   "_GeneratorNext": true,
   "_GeneratorThrow": true,
 
   // Only applicable to DataViews.
   "DataViewInitialize":true,
   "DataViewGetBuffer": true,
   "DataViewGetByteLength": true,
   "DataViewGetByteOffset": true,
-
-  // Only ever called internally.
-  "RunMicrotasks": true
 };
 
 var currentlyUncallable = {
   // We need to find a way to test this without breaking the system.
   "SystemBreak": true,
   // Inserts an int3/stop instruction when run with --always-opt.
   "_DebugBreakInOptimizedCode": true
 };
 
 function testNatives() {
   var allNatives = %ListNatives();
   var start = (allNatives.length >> 2)*2;
   var stop = (allNatives.length >> 2)*3;
   for (var i = start; i < stop; i++) {
     var nativeInfo = allNatives[i];
     var name = nativeInfo[0];
     if (name in knownProblems || name in currentlyUncallable)
       continue;
     print(name);
     var argc = nativeInfo[1];
     testArgumentCount(name, argc);
     testArgumentTypes(name, argc);
   }
 }
 
 testNatives();