[ChromeDriver] Chromedriver 2.40 release

Releasing new Chromedriver 2.40

Change-Id: Icca7d51d495fe4fe86aa3bc49c00d04eb636fa40
Reviewed-on: https://chromium-review.googlesource.com/1091234
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565379}

chrome/test/chromedriver/VERSION

-2.39
+2.40
+