Tracking bug for making AX node trees work. This is needed for both SingleProcessMash and out-of-process Mash.

High level design from meeting with dtseng:

* Ash is in charge of the desktop tree. It has an AutomationManagerAura or equivalent.
* "Remote apps" use AXRemoteHost to supply node trees to ash
* The browser is just a "remote app" like keyboard shortcut viewer
* Ash passes the node trees to the browser, which forwards them to the accessibility extension
* As a future optimization we might pass around mojo interface pointers so the remote app could talk directly to the browser, or directly to the extension

Things to do to make this happen:
* Either refactor AutomationManagerAura and move it to ash, or write something similar and new
* Kill Profile dependencies where possible
* Remove //ash dependencies from //chrome/browser accessibility code
* Deal with RenderViewHost / WebView embedding
* Support multiple remote hosts with different AX tree ids

See blocked-on bugs.