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This will avoid exposing in-progress zip file to the destination folder.
It will resolve the issues like:
- the zip file is synced to Drive with incomplete state, having multiple
  revisions
- when cancelling zip packing, the file appears in the trash of Drive

Bug: 714579,785096,785093,785086,797873
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I0b32515d5e896d87e1dee499f1eea40dd7a481f0
Reviewed-on: https://chromium-review.googlesource.com/844442
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547428}