Only delete old breakpad dumps if --clobber-old-results is given

BUG=none
R=dpranke@chromium.org,mkwst@chromium.org

Review URL: https://codereview.chromium.org/61853002

git-svn-id: svn://svn.chromium.org/blink/trunk@161453 bbb929c8-8fbe-4397-9dbb-9b2b20218538