Fix crash when running isolateserver download.py with --cache.
Fix crash when running isolateserver download.py with --cache.

Change-Id: I0547cbf82a96430b230239ae7e7a09660a4b61c5
Reviewed-on: https://chromium-review.googlesource.com/1077818
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py