Speculative fix for windows hangs in run-webkit-tests.

It looks like we may be seeing cases where trying to kill a content_shell
process doesn't actually kill everything. It's possible that passing the
/t flag to taskkill in addition to /f will help.

TBR=cpu@chromium.org
BUG=309787

Review URL: https://codereview.chromium.org/127293003

git-svn-id: svn://svn.chromium.org/blink/trunk@164647 bbb929c8-8fbe-4397-9dbb-9b2b20218538