[GTK][WK2] Implement LayerTreeHostGtk::pageBackgroundTransparencyChanged
https://bugs.webkit.org/show_bug.cgi?id=115094

Reviewed by Tim Horton.

The method was introduced in r149004 (and a stub implementation for the GTK port added in r149017).
Implement the method for the GTK port in the same manner it is implemented under the Mac port,
calling setContentsOpaque on the non-composited content layer.

* WebProcess/WebPage/gtk/LayerTreeHostGtk.cpp:
(WebKit::LayerTreeHostGtk::pageBackgroundTransparencyChanged):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149049 268f45cc-cd09-0410-ab3c-d52691b4dbfc