cros: Load pin assets on sign-in screen.

chrome/browser/resources/chromeos/login/custom_elements_login.html

+<!-- PIN keyboard is used outside of OOBE/login/lock so it includes the JS it
+     executes. This causes security errors if textually included here. -->
+<link rel="import" href="chrome://oobe/custom_elements/pin_keyboard.html">
+
 <include src="gaia_buttons.html">
 <include src="gaia_card.html">
 <include src="gaia_header.html">
 <include src="gaia_input_form.html">
 <include src="gaia_input.html">
 <include src="gaia_password_changed.html">
 <include src="hd-iron-icon.html">
 <include src="html-echo.html">
 <include src="offline_gaia.html">
 <include src="saml_confirm_password.html">
 <include src="saml_interstitial.html">
 <include src="throbber_notice.html">
 <include src="navigation_bar.html">
 <include src="unrecoverable_cryptohome_error_card.html">
 <include src="offline_ad_login.html">
 <include src="active_directory_password_change.html">
 <include src="arc_terms_of_service.html">
 <include src="oobe_buttons.html">
 <include src="oobe_dialog.html">
 <include src="oobe_reset.html">
 <include src="oobe_reset_confirmation_overlay.html">
 <include src="encryption_migration.html">
 
 <script src="chrome://oobe/custom_elements.js"></script>