[Metro] Focus the user manager's web contents after the widget is shown

Otherwise, on Ash, it seems to steal the initial focus away from the
widget.

BUG=453284
TEST=Start Chrome with --enable-new-avatar menu, and in Metro mode.
Choose "Switch Person" from the avatar menu. In the user manager, start
tabbing (without clicking on the user manager). This should work, and
tabbing should change the selected user pod.

Review URL: https://codereview.chromium.org/894723003

Cr-Commit-Position: refs/heads/master@{#314248}

chrome/browser/ui/views/profiles/user_manager_view.cc

@@ -171,9 +171,8 @@ void UserManagerView::Init(Profile* system_profile, const GURL& url) {
   if (rwhv)
     rwhv->SetBackgroundColor(profiles::kUserManagerBackgroundColor);
 
-  web_view_->RequestFocus();
-
   GetWidget()->Show();
+  web_view_->RequestFocus();
 }
 
 bool UserManagerView::AcceleratorPressed(const ui::Accelerator& accelerator) {