Make FileReaderLoader::start(Blob*)'s argument const reference.

To be consistent with the new method start(const Stream&) that is added
by https://codereview.chromium.org/18991017/.

BUG=169957

Review URL: https://chromiumcodereview.appspot.com/19002003

git-svn-id: svn://svn.chromium.org/blink/trunk@154117 bbb929c8-8fbe-4397-9dbb-9b2b20218538