Revert 177182 - ViewTest.ChangeNativeViewHierarchyFindRoots failure

Revert 177182 - ViewTest.ChangeNativeViewHierarchyFindRoots failure
> Finally rip the global StackingClient bandaid off.
> 
> - Removes the aura::client::SetStackingClient(StackingClient*) interface.
> - Moves the ash StackingController from a singleton owned by ash::Shell to one StackingController per RootWindow owned by the ash::RootWindowController. (Also removes a spurious delegate method, where every implementation creates the same object, including tests.)
> - Removes the global DesktopStackingClient and related interfaces.
> - Fix the ChromeViewsDelegate so that it still sets context in chromeos builds.
> - Rename content::ShellStackingClientAsh to content::MinimalAsh to reflect what it really does.
> 
> In addition, the following fix ups apply:
> 
> - Previously, WebContentsViewAura asserted that it needed a context window. Now if no context window is provided, it isn't added to an aura hierarchy. There are times when that context doesn't exist: various chromeos dialogs that directly invoke WebView and toplevel extension/app windows that are created from background pages that don't have contexts.
> 
> BUG=161882
> 
> Review URL: https://codereview.chromium.org/11829040

TBR=erg@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=177190