Add missing bower.json for third_party/Polymer

This allows "bower update" to work correctly by specifying dependencies.

Review URL: https://codereview.chromium.org/142863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245974 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245974