Settings WebUI: Add tabindex -1 in scrolling container to prevent tabstop.

This is necessary because of recent Blink changes at r608013.

Bug: 905999
Change-Id: I111e3631e4d832afb48a4161552fc27a1bbb9630
Reviewed-on: https://chromium-review.googlesource.com/c/1343197Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609546}

chrome/browser/resources/settings/settings_ui/settings_ui.html

@@ -78,7 +78,7 @@
         </template>
       </div>
     </cr-drawer>
-    <div id="container" class="no-outline">
+    <div id="container" class="no-outline" tabindex="-1">
       <settings-main id="main" prefs="{{prefs}}"
           toolbar-spinner-active="{{toolbarSpinnerActive_}}"
           page-visibility="[[pageVisibility_]]"