Make panel window's header color customizable in Ash.

Normal Chrome OS app window (ash::CustomFrameViewAsh) has an ability to change header color.
But if the type is 'panel', ash::PanelFrameView is used and it doesn't have the ability.
This CL adds the ability to change the header color for panel window.


Review URL:

Cr-Commit-Position: refs/heads/master@{#358005}
3 files changed