Add extension API test for FileSystem API.

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

+<script>
+  var fileSystem = null;
+
+  console.log("Requesting a filesystem...");
+  requestFileSystem(window.TEMPORARY, 100, getFileSystem, errorCallback);
+
+  function getFileSystem(fs) {
+    fileSystem = fs;
+    console.log("DONE requesting filesystem: " + fileSystem.name);
+    fileSystem.root.getDirectory('dir', {create:true},
+                                 directoryCallback, errorCallback);
+  }
+
+  function directoryCallback(directory) {
+    console.log("DONE creating directory: " + directory.path);
+    directory.getFile('file', {create:true}, fileCallback, errorCallback);
+  }
+
+  function fileCallback(file) {
+    console.log("DONE creating file: " + file.path);
+
+    // See if we get the same filesystem space in the tab.
+    console.log("Opening tab...");
+    chrome.tabs.create({
+      url: "tab.html"
+    });
+  }
+
+  function errorCallback(error) {
+    chrome.test.fail("Got unexpected error: " + error.code);
+  }
+</script>