Wires up immersive mode for chrome and mash

ash/mus/frame/README.md

Index: ash/mus/frame/README.md
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+This directory contains code needed for rendering and interacting with
+frames decorations in mash.

[James Cook, 2016/08/24 19:56:29]

nit: frames -> frame

[sky, 2016/08/24 20:11:14]

Done.

+
+### Immersive mode

[James Cook, 2016/08/24 19:56:29]

I would change this to "immersive fullscreen" and add a sort description of the
feature, something like:

Immersive fullscreen is a fullscreen mode similar to Mac OS presentation 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.

(And I like the idea of more README.md files!)

[sky, 2016/08/24 20:11:14]

I modified this slightly, but mostly kept it.

+
+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.