Refactor file_path_verifier.cc's UMA stat gathering.

Addressing post-commit comments from asvitkine.

BUG=299100
R=asvitkine@chromium.org, sail@chromium.org

Review URL: https://codereview.chromium.org/24532004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225522 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225522