[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 use of a plane view, in order to not draw any custom
window decoration and let that up to the native platform.

This PR needs the counter part PR for chromium-crosswalk. The chromium PR
changes desktop_root_window_host to request each platform to draw window
decoration.

BUG=https://crosswalk-project.org/jira/browse/XWALK-629