This is an evolving abstraction of the Browser (chrome/browser/ui/browser.h)
and BrowserList (chrome/browser/ui/browser_list.h) classes for use by
ChromeOS feature code.
The abstraction consists of two (C++) interfaces, BrowserDelegate &
BrowserController, and their implementation. Think of BrowserDelegate as an
abstract version of the existing Browser class and BrowserController as a
supervising entity that primarily creates or finds BrowserDelegate objects for
you.
Development is incremental and in the first phase we prioritize making these
classes the bottleneck for Browser access over them offering a clean API.
We aim to eventually move these interfaces out of chrome/browser/ash/ into chromeos/.