This also deletes downloaded test data files if their hashes are
incorrect to automatically retry the download on the next run.

R=jkummerow@chromium.org
TEST=test262

Review URL: https://chromiumcodereview.appspot.com/10080007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11322 ce2b1a6d-e550-0410-aec6-3dcde31c8c00