Add polymer-selector and polymer-selection to polymer.

These polymer UI components are needed for the PDF extension.

BUG=303491
R=bshe@chromium.org, koz@chromium.org

Review URL: https://codereview.chromium.org/141483005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246216 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246216