[Passwords] Bulk check banner alt attribute.

This CL adds alt="" to the banner <img> for accessibility purposes.

Bug: 1047726
Change-Id: Ib3e94edf8c1131726680e13ef61daf1dfec37ce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116231Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/master@{#752778}

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

@@ -116,7 +116,7 @@
           <source
               srcset="chrome://settings/images/password_check_neutral_dark.svg"
               media="(prefers-color-scheme: dark)">
-          <img id="banner"
+          <img id="banner" alt=""
               src="chrome://settings/images/password_check_neutral.svg">
         </picture>
       </div>