Improve work-around for importScripts bug.

Reverts "Hack to avoid intermidiate Chrome failures during tests."
(2b2c521213).

require.js uses importScript asynchronously, which activates the worker
GC bug in WebKit. This patch works around a bug in a way that is similar
in the upcoming (but not yet released) require.js 2.1.23

The advantage of the new work-around is that it allows the runtime to
garbage-collect idle Workers.

References:
- https://crbug.com/572225
- https://webkit.org/b/153317

src/worker_loader.js

@@ -15,6 +15,17 @@
 
 'use strict';
 
+//#if !PRODUCTION
+//// Patch importScripts to work around a bug in WebKit and Chrome 48-.
+//// See https://crbug.com/572225 and https://webkit.org/b/153317.
+self.importScripts = (function (importScripts) {
+  return function() {
+    setTimeout(function () {}, 0);
+    return importScripts.apply(this, arguments);
+  };
+})(importScripts);
+//#endif
+
 importScripts('../node_modules/requirejs/require.js');
 
 require.config({paths: {'pdfjs': '.'}});