Make filename processing a static method.
Make filename processing a static method.

This is in preparation to support file rename, as a source_filename needs to be
added and processed.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7809001

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@99185 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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