Fixes duplicate listing of third party libraries in credits

The party license generator can return duplicate paths when a path is both
defined in ADDITIONAL_PATHS and found through the recursive scan. This patch
makes sure an additional path is not reported multiple times if it has
already been found.

BUG=329448

Review URL: https://codereview.chromium.org/114723009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241834 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241834