If download from google storage gets terminated while extracting .tar.gz
archive, it won't be extracted on the next run which results in invalid
workspace. This patches places a flag file if extraction wasn't
completed. Such flag is checked on next execution and forces re-download
if found.

This does not fix currently broken workspaces.

R=dpranke@google.com, ehmladonado@chromium.org

Bug: 1189273
Change-Id: I4047d98c193493d9a0fe20a46043e1f8376e5c7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2774085Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>