Added combined test for Syncable File System to write file and then get that usage gets updated.

chrome/test/data/extensions/api_test/sync_file_system/write_file_then_get_usage/test.js

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var fileEntry;
+var fileSystem;
+var usageBeforeWrite;
+var testData = "12345";
+
+var testStep = [
+  function () {
+    chrome.syncFileSystem.requestFileSystem('drive', testStep.shift());
+  },
+  // Create empty file.
+  function(fs) {
+    fileSystem = fs;
+    fileSystem.root.getFile('Test.txt', {create: true}, testStep.shift(),
+        errorHandler);
+  },
+  function(entry) {
+    fileEntry = entry;
+    testStep.shift()();
+  },
+  // Record usage before write.
+  function() {
+    chrome.syncFileSystem.getUsageAndQuota(fileSystem, testStep.shift());
+  },
+  function(storageInfo) {
+    usageBeforeWrite = storageInfo.usage_bytes;
+    testStep.shift()();
+  },
+  // Write a known number of bytes.
+  function() {
+    fileEntry.createWriter(testStep.shift(), errorHandler);
+  },
+  function (fileWriter) {
+    fileWriter.onwriteend = function(e) {
+      testStep.shift()();
+    };
+    fileWriter.onerror = function(e) {
+      chrome.test.fail('Write failed: ' + e.toString());
+    };
+    fileWriter.seek(fileWriter.length);
+    var blob = new Blob([testData], {type: "text/plain"});
+    fileWriter.write(blob);
+  },
+  // Check the meta data for updated usage.
+  function() {
+    fileEntry.getMetadata(testStep.shift(), errorHandler);
+  },
+  function (metadata) {
+    chrome.test.assertEq(testData.length, metadata.size);
+    testStep.shift()();
+  },
+  // Check global usage was updated.
+  function() {
+    chrome.syncFileSystem.getUsageAndQuota(fileSystem, testStep.shift());
+  },
+  function(storageInfo) {
+    var usageAfterWrite = storageInfo.usage_bytes;
+    chrome.test.assertEq(testData.length, usageAfterWrite - usageBeforeWrite);
+    chrome.test.succeed();
+  }
+];
+
+function errorHandler() {
+  chrome.test.fail();
+}
+
+chrome.test.runTests([
+  testStep[0]
+]);
+