use storage.googleapis for testdata download

replace downloads.webmproject.org with the canonical
storage.googleapis.com/... form. this appears less likely to fail when
dealing with multiple concurrent connections.

original change in libvpx:
22f7aca09 use storage.googleapis for testdata download

Change-Id: Ie1bc329fecb336f089a662451b8ebf0c0d9dc2d6
1 file changed