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

This reverts commit 42c0440f.

Reason for revert: This causes https://crbug.com/908775, and the
Blink changes that were the reason for the change have been
rolled back for now.

Original change's description:
> 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/1343197
> Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
> Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609546}

TBR=dpapad@chromium.org,rbpotter@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 905999, 908775
Change-Id: Ie14d8cbea36dcb91d03e8e7ab7b2fdd5010fcc10
Reviewed-on: https://chromium-review.googlesource.com/c/1354235Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611906}

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

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