Remove use of TarFile as context manager.
Remove use of TarFile as context manager.

It seems that this is only available in python 2.7.

Review URL: https://codereview.chromium.org/11886080

git-svn-id: 84b3587ce119b778414dcb50e18b7479aacf4f1d@662 7dad1e8b-422e-d2af-fbf5-8013b78bd812

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