[Passwords] Add alt attribute to leak check banner image

An alt="" tag on images hide them from screen readers. Since the banner
on the leak check results page carries no additional information, it can
and should be skipped.

Bug: 1047726
Change-Id: I64299104ab86db6510fb4fcd199cf6d9f5c5c4b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120578Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753231}

chrome/browser/resources/settings/autofill_page/password_check.html

@@ -62,7 +62,7 @@
       <picture>
         <source srcset="[[bannerImageSrc_(1, status_)]]"
                 media="(prefers-color-scheme: dark)">
-        <img id="bannerImage" src="[[bannerImageSrc_(0, status_)]]">
+        <img id="bannerImage" src="[[bannerImageSrc_(0, status_)]]" alt="">
       </picture>
     </template>