Enable Aura XWalk build by default

Enabled Aura by default for XWalk Linux and Windows builds, also enabled window frame and window header for XWalk Aura windows on Linux desktop.

The NativeAppWindowViews::CreateNonClientFrameView is now overridden only for Windows in order to make visible Aura window frame&header on Linux desktop (NativeAppWindowViews::CreateNonClientFrameView had been overridden to show OS window decoration instead chrome-style UI, however the used views::NativeFrameView works only on Windows).
Merge pull request #838 from pozdnyakov/Aura_by_default

Enable Aura build by default