[Windows] Enable aura on Windows

The origin MACRO protection for XWalkViewsDelegate
is "#if !defined(OS_WIN) && defined(USE_AURA)", which
is not appropriate on Windows after Windows build also
switches to use aura.

Change the condition to "#if defined(USE_AURA)" and
implement the missing abstract function on Windows.
Merge pull request #987 from wang16/windows_aura

[Windows] Enable aura on Windows