Make a pref test more robust.
Make a pref test more robust.

Ignore errors when cleaning up.

shutil.rmtree() works by os.listdir() followed by rm'ing each entry.
If a file goes missing after os.listdir(), a os.remove will be attempted for it resulting in an exception.

In this case it might happen so (in once in a blue moon) because chrome creates temp files while downloading and cleans them up.

Review URL: http://codereview.chromium.org/3143017

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@56149 0039d316-1c4b-4281-b951-d872f2087c98

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