Add test case for ::selection:window-inactive

Add test case for ::selection:window-inactive
https://bugs.webkit.org/show_bug.cgi?id=79623

Reviewed by Beth Dakin.

Add reftest to ensure that we don't regress inactive selection color.

For completeness, support for inactive selection color was added in
<http://trac.webkit.org/changeset/41094> (<rdar://problem/6077775>).

* fast/selectors/selection-window-inactive-expected.html: Added.
* fast/selectors/selection-window-inactive.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@109045 268f45cc-cd09-0410-ab3c-d52691b4dbfc