Add isWritableFileEntry to the fileSystem API

chrome/test/data/extensions/api_test/file_system/test_util.js

Index: chrome/test/data/extensions/api_test/file_system/test_util.js
diff --git a/chrome/test/data/extensions/api_test/file_system/test_util.js b/chrome/test/data/extensions/api_test/file_system/test_util.js
index 479ef7fbd9d05423ef57bb770c4e5e39af1bb901..ecefc00ace8eab4080f8cb5546223a855b77bc26 100644
--- a/chrome/test/data/extensions/api_test/file_system/test_util.js
+++ b/chrome/test/data/extensions/api_test/file_system/test_util.js
@@ -7,6 +7,13 @@
 
 function checkEntry(entry, expectedName, isNew, shouldBeWritable) {
   chrome.test.assertEq(expectedName, entry.name);
+
+  // Test that we are writable (or not), as expected.
+  chrome.fileSystem.isWritableFileEntry(entry, chrome.test.callbackPass(
+      function(isWritable) {
+    chrome.test.assertEq(isWritable, shouldBeWritable);
+  }));
+
   // Test that the file can be read.
   entry.file(chrome.test.callback(function(file) {
     var reader = new FileReader();