Fix unwanted shadow around PIN dialog in OOBE

This removes the border and shadow around the smart card PIN dialog (the
<security-token-pin> Polymer element) when it's shown during OOBE.

Bug: 1024251
Test: powerwash Chromebook, enroll under policies with smart card support, start smart card login, check that the PIN dialog has no border or shadow
Change-Id: I4355663bbc4f6de60512876282d7fbbf0e82e183
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028772Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736835}

chrome/browser/resources/chromeos/login/screen_gaia_signin.js

@@ -722,6 +722,7 @@ Polymer({
 
     this.$['offline-ad-auth'].onBeforeShow();
     this.$['signin-frame-dialog'].onBeforeShow();
+    this.$.pinDialog.onBeforeShow();
   },
 
   /**