[UI] Make CrossWalk Aura use native window decoration.

Current CrossWalk Aura port (linux and window) draws window decoration by
itself, so look-and-feel is very different to native ui.
We need to delegate window decoration to each platform.

Widget::CreateNonClientFrameView() gives a chance to an embedder to decorate window
frame. If an embedder does not do anything, Widget makes aura default window
decoration. XWalk makes a just plain view to not draw anything, because XWalk
relies on each platform to decorate window frame.

This patch needs the counter part patch for chromium-crosswalk. The chromium
patch changes desktop_root_window_host to request each platform to draw window
decoration.

BUG=https://crosswalk-project.org/jira/browse/XWALK-629