Move everything under ui/ folder to ui namespace.

Eventually, we should move ipc/, ui/, desktop_aura/ to their own namespace
as rest of chromium code. This CL moves everything under ui/ folder to ui
namespace.
Move everything under desktop_aura/ to view namespace.
Move impl/desktop_aura/ to ui/
IPC: Move DisplayChannel and host to content namespace.

DisplayChannel and Host are very specific to our needs as we do everything
related to a window system in GPU process. This has to be really inbuilt in
content layer. Now, both the classes are moved to content namespace and
eventually should be build as part of content target.