[Chromedriver] Release Chromedriver version 2.31

Change-Id: Ie44eb482868022111fc52ff490702e0a97d0f4d4
Reviewed-on: https://chromium-review.googlesource.com/580053Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488756}

chrome/test/chromedriver/VERSION

-2.30
+2.31