ash uses this to look up BrowserContext (Profile) on a per-window basis:
content::BrowserContext* GetActiveBrowserContext() override;
content::BrowserContext* GetBrowserContextByIndex(
ash::UserIndex index) override;
content::BrowserContext* GetBrowserContextForWindow(
aura::Window* window) override;
content::BrowserContext* GetUserPresentingBrowserContextForWindow(
aura::Window* window) override;
I think this is used for multiprofile desktop switching (deciding which window to show for which user on which desktop).
Comment 1 by jamescook@chromium.org
, Apr 19 2018