* refactor: make RootView::window_ a const raw_ref
The Chromium C++ style guide says "prefer const raw_ref<T> whenever
the held pointer will never be null," so let's do that.
* refactor: make RootView::main_view_ a const raw_ref
> The Chromium C++ style guide says "prefer const raw_ref<T> whenever
> the held pointer will never be null," so let's do that.
* refactor: aggregate RootView::last_focused_view_tracker_
RootView already owns it, so aggregate it