Use WorkerPool for unzipping recovery image to avoid blocking FILE thread for long time.
Use WorkerPool for unzipping recovery image to avoid blocking FILE thread for long time.

BUG=chromium-os:25316
TEST=chrome://imageburner works


Review URL: http://codereview.chromium.org/9270040

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@119205 0039d316-1c4b-4281-b951-d872f2087c98

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