Hack to avoid intermidiate Chrome failures during tests.

Remove when https://code.google.com/p/chromium/issues/detail?id=572225 is fixed.

src/display/api.js

@@ -1253,7 +1253,16 @@ var PDFWorker = (function PDFWorkerClosure() {
           // https://bugzilla.mozilla.org/show_bug.cgi?id=683280
           var worker = new Worker(workerSrc);
           var messageHandler = new MessageHandler('main', 'worker', worker);
+//#if !PRODUCTION
+          // Don't allow worker to be destroyed by Chrome, see:
+          // https://code.google.com/p/chromium/issues/detail?id=572225
+          var jsWorkerId = '_workerKungfuGrip_' + Math.random();
+          window[jsWorkerId] = worker;
+//#endif
           messageHandler.on('test', function PDFWorker_test(data) {
+//#if !PRODUCTION
+            delete window[jsWorkerId];
+//#endif
             if (this.destroyed) {
               this._readyCapability.reject(new Error('Worker was destroyed'));
               messageHandler.destroy();