[GTK] Several labels are white instead of black

[GTK] Several labels are white instead of black
https://bugs.webkit.org/show_bug.cgi?id=137803

Reviewed by Martin Robinson.

Recent GTK+ versions require to explicitly set the state before
getting a color.

* rendering/RenderThemeGtk.cpp:
(WebCore::styleColor): Helper function to get a color from a
GtkStylecontext that explicitly sets the state before getting the color.
(WebCore::RenderThemeGtk::platformActiveSelectionBackgroundColor): Use styleColor().
(WebCore::RenderThemeGtk::platformInactiveSelectionBackgroundColor): Ditto.
(WebCore::RenderThemeGtk::platformActiveSelectionForegroundColor): Ditto.
(WebCore::RenderThemeGtk::platformInactiveSelectionForegroundColor): Ditto.
(WebCore::RenderThemeGtk::platformActiveListBoxSelectionBackgroundColor): Ditto.
(WebCore::RenderThemeGtk::platformInactiveListBoxSelectionBackgroundColor): Ditto.
(WebCore::RenderThemeGtk::platformActiveListBoxSelectionForegroundColor): Ditto.
(WebCore::RenderThemeGtk::platformInactiveListBoxSelectionForegroundColor): Ditto.
(WebCore::RenderThemeGtk::systemColor): Ditto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@174929 268f45cc-cd09-0410-ab3c-d52691b4dbfc