Always reload cache entries for a downloadToFile request.

If a downloadToFile (e.g., XHR with responseType 'blob') request finds a cache
entry, it should always reload it; the caller is expecting the data in a
different form than the cache entry, so we hit assertions.

BUG=338936
TEST=http/tests/xmlhttprequest/blob-response-type-warm-cache.html

Review URL: https://codereview.chromium.org/145123009

git-svn-id: svn://svn.chromium.org/blink/trunk@166075 bbb929c8-8fbe-4397-9dbb-9b2b20218538