Add 'Windows Kits\8.1\Debuggers' to CDB search paths.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/399953002

git-svn-id: svn://svn.chromium.org/blink/trunk@178382 bbb929c8-8fbe-4397-9dbb-9b2b20218538