Clean up browser tests in Files.app.

This patch cleans up tests on the Javascript side for Files.app:
- Introduce a StepRunner, which runs multi-step test cases.
- Add checking for Javascript errors after each test case.
- Add missing comments.

TEST=browser_tests
BUG=244327

Review URL: https://chromiumcodereview.appspot.com/15809007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203123 0039d316-1c4b-4281-b951-d872f2087c98

chrome/test/data/extensions/api_test/file_manager_browsertest/background.js

@@ -28,6 +28,54 @@ function callRemoteTestUtil(func, appId, args, callback) {
       callback);
 }
 
+/**
+ * Executes a sequence of test steps.
+ * @constructor
+ */
+function StepsRunner() {
+  /**
+   * List of steps.
+   * @type {Array.function>}
+   * @private
+   */
+  this.steps_ = [];
+}
+
+/**
+ * Creates a StepsRunner instance and runs the passed steps.
+ */
+StepsRunner.run = function(steps) {
+  var stepsRunner = new StepsRunner();
+  stepsRunner.run_(steps);
+};
+
+StepsRunner.prototype = {
+  /**
+   * @return {function} The next closure.
+   */
+  get next() {
+    return this.steps_.shift();
+  }
+};
+
+/**
+ * Runs a sequence of the added test steps.
+ * @type {Array.<function>} List of the sequential steps.
+ */
+StepsRunner.prototype.run_ = function(steps) {
+  this.steps_ = steps.slice(0);
+
+  // An extra step which acts as an empty callback for optional asynchronous
+  // calls in the last provided step.
+  this.steps_.push(function() {});
+
+  this.steps_ = this.steps_.map(function(f) {
+    return chrome.test.callbackPass(f.bind(this));
+  }.bind(this));
+
+  this.next();
+};
+
 chrome.test.runTests([
   // Waits for the C++ code to send a string identifying a test, then runs that
   // test.