Increase padding to SAML smart card PIN error

This moves the error label a bit down in the SAML smart card PIN dialog,
in conformance to the UI feedback.

As was clarified by the designers, the padding should be 4px to the
upper bound of the text box, not to the upmost pixel of the font.
Conceptually, the error message should look centered between the input
field and the keypad below.

This CL moves the error label by 3 pixels downwards, in accordance to
this corrected understanding of the requirements.

Bug: 1024251
Test: start SAML login via a smart card, proceed to PIN dialog, take a screenshot, verify the distance
Change-Id: I3e2aa96151254595424010e4db33673f5ddd0d31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089757Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747339}

chrome/browser/resources/chromeos/login/security_token_pin.css

@@ -19,7 +19,7 @@
 #errorContainer {
   color: var(--google-red-600);
   font-size: 12px;
-  padding: 1px 0 3px;
+  padding: 4px 0 0;
   text-align: center;
   width: 100%;
 }