Add extension API test for FileSystem API.

chrome/test/data/extensions/api_test/fileapi/tab.html

+<script>
+var fileSystem = null;
+
+function errorCallback(error) {
+  chrome.test.fail("Got unexpected error: " + error.code);
+}
+
+function successCallback(entry) {
+  chrome.test.succeed();
+}
+
+function successEntryCallback(entry) {
+  fileSystem.root.getDirectory('dir', {create:false},
+      function(directory) {
+        // Do clean-up.  (Assume the tab won't be reloaded in testing.)
+        directory.removeRecursively(successCallback, errorCallback);
+      }, errorCallback);
+}
+
+chrome.test.runTests([function tab() {
+  console.log("Requesting a filesystem...");
+  requestFileSystem(window.TEMPORARY, 100, function(fs) {
+    fileSystem = fs;
+    // See if we get the same filesystem image.
+    console.log("DONE requesting filesystem: " + fileSystem.name);
+    fileSystem.root.getFile('dir/file', {create:false},
+                            successEntryCallback, errorCallback);
+  }, errorCallback);
+}]);
+</script>