Add tests for chrome.fileSystem app API.

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

+chrome.test.runTests([
+  function saveFile() {
+    chrome.fileSystem.chooseFile({type: 'saveFile'}, chrome.test.callbackPass(
+        function(entry) {
+      chrome.test.assertEq('save_existing.txt', entry.name);
+      // Test that the file can be readt.
+      entry.file(chrome.test.callback(function(file) {
+        var reader = new FileReader();
+        reader.onloadend = chrome.test.callbackPass(function(e) {
+          chrome.test.assertEq(reader.result.indexOf("Can you see me?"), 0);
+          // Test that we can write to the file.
+          entry.createWriter(function(fileWriter) {
+            fileWriter.onwriteend = chrome.test.callback(function(e) {
+              if (fileWriter.error) {
+                chrome.test.fail("Error writing to file: " +
+                                 fileWriter.error.toString());
+              } else {
+                // Get a new entry and check the data got to disk.
+                chrome.fileSystem.chooseFile(chrome.test.callbackPass(
+                    function(readEntry) {
+                  readEntry.file(chrome.test.callback(function(readFile) {
+                    var readReader = new FileReader();
+                    readReader.onloadend = function(e) {
+                      chrome.test.assertEq(readReader.result.indexOf("HoHoHo!"),
+                                           0);
+                      chrome.test.succeed();
+                    };
+                    readReader.onerror = function(e) {
+                      chrome.test.fail('Failed to read file after write.');
+                    };
+                    readReader.readAsText(readFile);
+                  }));
+                }));
+              }
+            });
+            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);
+      }));
+    }));
+  }
+]);