Wires up immersive mode for chrome and mash

ash/mus/frame/README.md

+This directory contains code needed for rendering and interacting with
+frame decorations in mash.
+
+### Immersive fullscreen
+
+Immersive fullscreen is a specialized fullscreen mode. In it the user
+can move the mouse to the top of the screen and the window header
+will slide down on top of the fullscreen window contents.
+
+There are two distinct ways for immersive mode to work in mash:
+
+1. Mash handles it all. This is the default. In this mode a separate
+ui::Window is created for the reveal of the title area. HeaderView is used to
+render the title area of the reveal in the separate window. The client
+does nothing special here.
+
+2. The client takes control of it all (as happens in chrome). To
+enable this the client sets kDisableImmersive_Property on the window. In this
+mode the client creates a separate window for the reveal (similar to
+1). The reveal window is a child of the window going into immersive
+mode. Mash knows to paint window decorations to the reveal window by way of
+the property kRendererParentTitleArea_Property set on the parent
+window. The client runs all the immersive logic, including positioning
+of the reveal window.
+
+In both cases DetachedTitleAreaRenderer and HeaderView is used to
+render the title area. The difference is in the first case mash
+creates the window, in the second case the child does (and tells mash
+about it). And of course in the second case the client runs all the
+immersive logic.