Use conditionals and hooks_os for download_from_google_storage hooks

This should be a no-op, and is part of a migration path
away from hooks_os.

It's also one of the smallest experiments with conditionals
in chromium/src.

Bug: 570091
Change-Id: I9d582a6703c64a4ab37bb063ebebe85a7b41b4b5
Reviewed-on: https://chromium-review.googlesource.com/671383
Reviewed-by: Michael Moss <[email protected]>
Reviewed-by: Aaron Gable <[email protected]>
Commit-Queue: Michael Moss <[email protected]>
Cr-Commit-Position: refs/heads/master@{#502706}
1 file changed