Fix tab support in Assistant settings

This is a temporary fix.
The real fix will be to move away from the Content Service and back to
|views::WebView|. This is tracked in b/146351046.

   - Embedded UI:
       - TAB and shift-TAB works for settings.
       - TAB and shift-TAB works for webview based Assistant response
       (to the query |weather|).
       - Open Settings, click on other window, click back on settings,
       verify TAB and shift-TAB still work.
   - Standalone UI:
       - TAB and shift-TAB works for settings.
       - TAB and shift-TAB works for webview based Assistant response
       (to the query |weather|).

Bug: 145213680
Change-Id: If0f39d4911409f177fb1f93525205ad7392bdd35
Tests: Manually tested that:
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1979633
Reviewed-by: Xiaohui Chen <[email protected]>
Commit-Queue: Jeroen Dhollander <[email protected]>
Cr-Commit-Position: refs/heads/master@{#727225}

ash/assistant/ui/assistant_web_view.cc

@@ -86,8 +86,19 @@ void AssistantWebView::OnFocus() {
 }
 
 void AssistantWebView::AboutToRequestFocusFromTabTraversal(bool reverse) {
-  if (contents_)
+  if (contents_) {
+    // TODO(b/146351046): Temporary workaround for b/145213680. Should be
+    // removed once we have moved off of the Content Service (tracked in
+    // b/146351046).
+    base::SequencedTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::BindOnce(
+                       [](base::WeakPtr<AssistantWebView> view) {
+                         if (view)
+                           view->GetFocusManager()->ClearFocus();
+                       },
+                       weak_factory_.GetWeakPtr()));
     contents_->FocusThroughTabTraversal(reverse);
+  }
 }
 
 void AssistantWebView::InitLayout() {