If a preferences file references a theme which is from an old version, or whose images have failed to process correctly, drop back to the default theme.
If a preferences file references a theme which is from an old version, or whose images have failed to process correctly, drop back to the default theme.

BUG= http://crbug.com/20263
TEST= Use a preferences file from an older version of Chrome that has a theme installed, and open Chrome.  Observe that Chrome opens using the default theme.

Review URL: http://codereview.chromium.org/175013

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@24830 0039d316-1c4b-4281-b951-d872f2087c98

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