Add autoStep helper to FileManager integration test extensions

Add autoStep() helper for step-by-step mode. Typing it at the devtools
console (or evaluating it in other remote inspection tools) causes all
the test steps to run automatically in step by step mode [1].

[1] out/Release/browser_tests --enable-file-manager-step-by-step-tests

Tbr: lucmult
Bug: 863800,836254
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I5923f2ad0cc4fcc98e20e7bdf66e9ad504a4f9cf
Reviewed-on: https://chromium-review.googlesource.com/1137968Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575220}

ui/file_manager/integration_tests/remote_call.js

@@ -4,6 +4,19 @@
 
 'use strict';
 
+/**
+ * When step by step tests are enabled, turns on automatic step() calls. Note
+ * that if step() is defined at the time of this call, invoke it to start the
+ * test auto-stepping ball rolling.
+ */
+function autoStep() {
+  window.autostep = window.autostep || false;
+  if (!autostep)
+    autostep = true;
+  if (autostep && typeof window.step == 'function')
+    window.step();
+}
+
 /**
  * Class to manipulate the window in the remote extension.
  *
@@ -45,11 +58,16 @@ RemoteCall.prototype.callRemoteTestUtil =
     return new Promise(function(onFulfilled) {
       console.info('Executing: ' + func + ' on ' + appId + ' with args: ');
       console.info(args);
-      console.info('Type step() to continue...');
-      window.step = function() {
-        window.step = null;
+      if (window.autostep !== true) {
+        console.info('Type step() to continue...');
+        window.step = function() {
+          window.step = null;
+          onFulfilled(stepByStep);
+        };
+      } else {
+        console.info('Auto calling step() ...');
         onFulfilled(stepByStep);
-      };
+      }
     });
   }).then(function(stepByStep) {
     return new Promise(function(onFulfilled) {