cros_sdk: do not resume transfers on file:// urls
cros_sdk: do not resume transfers on file:// urls

This currently breaks chromiumos-sdk bot, which tests the newly created
SDK by passing a tarball to cros_sdk, but never changes the filename.

BUG=chromium-os:19447
TEST=below
1) run it on file:// link, see always redownload the whole thing
2) run it on a remote URL, see it always resume downloads

Change-Id: I07ff4f8eea73b2865a6268862bf3a7577f2a96f1
Reviewed-on: http://gerrit.chromium.org/gerrit/6381
Reviewed-by: Anush Elangovan <be3ea4b32dfea400cfb02e20cf45a400ef235334@google.com>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,coreos/chromite