Other notable changes:
* Removed WindowTreeHostFactory as this is no longer necessary.
* Moved RootWindowTransformer to ash/host
* Removed Set/GetInsets which are no longer necessary (a code in SetInsets is now a part of
SetRootWindowTransformer)
* Added TransformerHelper that implements common behavior to transform root window for AshWindowTreeHost
* Added DEPS files
* Updated metro_viewer code to make sure it creates ash::AshRemoteWindowHostTree

A few more simplifications to come:
* Simplify RootWindowTransformer
* Eliminate EnvObserver in AshWindowTreeHostX11

I'll also work on DEPS reorg to make it more explicit under ash/

BUG=355771
TEST=No functional change. ui/aura/window_tree_host_x11_unittests has been moved to ash/host/ash_window_tree_host_x11_unittests

Review URL: https://codereview.chromium.org/201573015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263192 0039d316-1c4b-4281-b951-d872f2087c98