Add button role to interstitial debugging information toggle

As discussed with a11y team in comments 29 and 31 crbug.com/1061611, it
is desirable that the net error code in SSL interstitials (which can be
clicked to show the certificate information) has a button role for
screen readers and other accessibility devices. This adds the role when
the error code is clickable.

Bug: 1061611
Change-Id: I02ac3ba3253fae3fcecff09da5fb7b683ba09258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582469
Commit-Queue: Carlos IL <carlosil@chromium.org>
Commit-Queue: Joe DeBlasio <jdeblasio@chromium.org>
Auto-Submit: Carlos IL <carlosil@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836282}

components/security_interstitials/core/browser/resources/ssl.js

@@ -21,5 +21,6 @@ function setupSSLDebuggingInfo() {
   }
 
   $('error-code').addEventListener('click', toggleDebuggingInfo);
+  $('error-code').setAttribute('role', 'button');
   $('error-code').setAttribute('aria-expanded', false);
 }