Fix cluster-fuzz regression with Workers on mips.debug

BUG=chromium:503698
R=jochen@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1208573003

Cr-Commit-Position: refs/heads/master@{#29261}

src/d8.cc

@@ -1725,8 +1725,8 @@ void Worker::ExecuteInThread() {
         }
       }
     }
+    Shell::CollectGarbage(isolate);
   }
-  Shell::CollectGarbage(isolate);
   isolate->Dispose();
 }
 

test/mjsunit/regress/regress-crbug-503698.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --invoke-weak-callbacks
+
+if (this.Worker) {
+  function __f_1() {}
+  var __v_6 = new Worker(__f_1);
+}