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

R=rossberg@chromium.org, rossberg
BUG=347532

Review URL: https://codereview.chromium.org/181013008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19588 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -14685,7 +14685,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_SetMicrotaskPending) {
 RUNTIME_FUNCTION(MaybeObject*, Runtime_RunMicrotasks) {
   HandleScope scope(isolate);
   ASSERT(args.length() == 0);
-  Execution::RunMicrotasks(isolate);
+  if (isolate->microtask_pending())
+    Execution::RunMicrotasks(isolate);
   return isolate->heap()->undefined_value();
 }
 

test/mjsunit/fuzz-natives-part3.js

@@ -217,9 +217,6 @@ var knownProblems = {
   "DataViewGetBuffer": true,
   "DataViewGetByteLength": true,
   "DataViewGetByteOffset": true,
-
-  // Only ever called internally.
-  "RunMicrotasks": true
 };
 
 var currentlyUncallable = {