Commander: refocus input when invoked

If a user selects a Commander option with the mouse, the text input loses focus. Since we don't reload the page when Commander is next invoked, the `autofocus` attribute on the input does nothing and the input starts unfocused.

This change adds an explicit focus when initializing the UI.

Bug: 1014639
Change-Id: Ic2dc86554d8956512a30bc08bc1268d68598d9c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115485


Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#914752}