Have Slider paint its background behind its visuals
Slider currently paints its control before calling View::OnPaint. If a background or border is set they will paint over the actual slider. Moving the call to View::OnPaint to before the custom painting in Slider.
TEST=Manual testing on a development branch. A background was set on a Slider in the SystemTray. The background no longer obscurs the slider.
BUG=447213
Review URL: https://codereview.chromium.org/
855023005
Cr-Commit-Position: refs/heads/master@{#312441}