Cancel on-going scan when change directory is requested.

BUG=477554
TEST=manually tested as described in the issue.

Review URL: https://codereview.chromium.org/1086353002

Cr-Commit-Position: refs/heads/master@{#325424}

ui/file_manager/file_manager/foreground/js/directory_model.js

@@ -924,6 +924,10 @@ DirectoryModel.prototype.changeDirectoryEntry = function(
   this.changeDirectorySequence_++;
   this.clearSearch_();
 
+  // If there is on-going scan, cancel it.
+  if (this.currentDirContents_.isScanning())
+    this.currentDirContents_.cancelScan();
+
   this.directoryChangeQueue_.run(function(sequence, queueTaskCallback) {
     this.fileWatcher_.changeWatchedDirectory(
         dirEntry,