[cros] Add user image transparency support.
[cros] Add user image transparency support.

BUG=chromium-os:23281
TEST=Manual: see bug description

Review URL: http://codereview.chromium.org/8923004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@114045 0039d316-1c4b-4281-b951-d872f2087c98

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