Add tests for chrome.fileSystem app API.

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

+chrome.test.runTests([
+  function openFile() {
+    chrome.fileSystem.chooseFile(chrome.test.callbackPass(function(entry) {
+      chrome.test.assertEq('open_existing.txt', entry.name);
+      // Test that the file can be read.
+      entry.file(chrome.test.callback(function(file) {
+        var reader = new FileReader();
+        reader.onloadend = chrome.test.callback(function(e) {
+          chrome.test.assertEq(reader.result.indexOf("Can you see me?"), 0);
+          // Test that we cannot write to the file.
+          entry.createWriter(chrome.test.callback(function(fileWriter) {
+            fileWriter.onwriteend = chrome.test.callback(function(e) {
+              if (fileWriter.error)
+                chrome.test.succeed();
+              else
+                chrome.test.fail("No error while writing without write access");
+            });
+            var blob = new Blob(['HoHoHo!'], {type: 'text/plain'});
+            fileWriter.write(blob);
+          }));
+        });
+        reader.onerror = chrome.test.callback(function(e) {
+          chrome.test.fail("Error reading file contents.");
+        });
+        reader.readAsText(file);
+      }));
+    }));
+  }
+]);