TarballCache: support fetching over gs://

We have helper modules for downloading files via gs://, so add support
to the current fetching logic in TarballCache to do just that.

BUG=chromium:322693
TEST=`upload_symbols gs://chromeos-image-archive/x86-generic-incremental/R34-5428.0.0-b16252/debug.tgz` downloads & unpacks
TEST=`./buildbot/run_tests` passes

Change-Id: Iae1a99605776db77c904857916e2576354ae01d9
Reviewed-on: https://chromium-review.googlesource.com/185565
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed