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This change uses the tempfile library to manage temporary named files
instead of using a DIY try/finally approach.

Bug: 832849
Change-Id: I439ff29edfb0e8791202604d56f891827d79f0a2
Reviewed-on: https://chromium-review.googlesource.com/1029212Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553908}