Expose FileList in Worker contexts.

LayoutTests/fast/files/workers/resources/worker-read-file-async.js

 importScripts("../../resources/read-common.js", "../../resources/read-file-test-cases.js", "worker-read-common.js");
 
+onmessage = function(event) {
+    var testFiles = event.data;
+    log("Received files in worker");
+    if (event.data[0] instanceof FileList) {
+        log("Received a FileList");
+        // The tests expects a name => File map; construct the
+        // mapping from the FileList and the test names that
+        // were posted (as a pair, using an Array.)
+        var files = event.data[0];
+        var names = event.data[1];
+        var tests = {};
+        for (var i = 0; i < files.length; i++)
+            tests[names[i]] = files[i];
+
+        testFiles = tests;
+    }
+    runNextTest(testFiles);
+};
+
 function isReadAsAsync()
 {
     return true;
 }
-