update.py: Fix a TODO that is easy to fix after DownloadAndUnpack.
update.py: Fix a TODO that is easy to fix after DownloadAndUnpack.

BUG=494442
R=hans@chromium.org

Review URL: https://codereview.chromium.org/1471963005 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#361731}

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