Update the AppPack cache directory.
Update the AppPack cache directory.

BUG=chromium-os:25463
TEST=The AppPack is cached in /var/cache/app_pack


Review URL: http://codereview.chromium.org/9751010

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@127746 0039d316-1c4b-4281-b951-d872f2087c98

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