Add .bat wrapper script for run-webkit-tests.

This makes easier for Windows developers to run this script on Windows
environment.

BUG=338338
TEST=Tools/Scripts/run-webkit-tests.bat on Windows
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/590743002

git-svn-id: svn://svn.chromium.org/blink/trunk@182432 bbb929c8-8fbe-4397-9dbb-9b2b20218538