WebUI Settings: Fix non-scaling banner

Add img width to settings/content style to properly scale banner

Bug: 1096441
Change-Id: Ia364223860c88f3aab1dbd273037fb6bba49a274
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252001Reviewed-by: Theodore Olsauskas-Warren <sauski@google.com>
Commit-Queue: Sean Harrison <harrisonsean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779965}

chrome/browser/resources/settings/site_settings_page/site_settings_page.html

@@ -2,6 +2,10 @@
       .no-min-height {
         min-height: 0;
       }
+
+      img {
+        width: 100%;
+      }
     </style>
     <template is="dom-if" if="[[!privacySettingsRedesignEnabled_]]">
       <div class="cr-row first no-min-height">