Whitelist chromite/third_party/poster. It's MIT licensed.

BUG=326117
TBR=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/106413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238924 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238924