[d8 Workers] Add max worker count, throw an exception if too many.

test/mjsunit/regress/regress-crbug-518748.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) {
-  var __v_6 = new Worker('');
+  function f(a) {
+    assertThrows(function() {
+      for (var i = 0; new Worker('onmessage = function() {};'); i++) a[i] = i;

[Michael Starzinger, 2015/08/10 22:01:23]

The regression test is kind of brittle, because if the workers finish fast
enough, there will never be more than 100 of them running in parallel. We need
some more reliable way to test that. Also a loop that doesn't run infinitely but
is bounded somehow would be better.

[binji, 2015/08/10 22:09:47]

Workers are not reaped until the main thread is finished, so this should not be
a problem. But if someone decided to fix that, you're right. In any case, there
needs to be a better test because this is failing on multiple bots as-is. :)

[Michael Starzinger, 2015/08/10 22:14:41]

Ah, got it. My bad, I was assuming the {workers} list in d8.cc would be emptied
out whenever a worker terminated. But yes, as you mentioned, for some reason
some try-bots still fail on this test, so it's brittle for other reasons.

+    });
+  }
+  f([]);
 }