Gtk3: Allow theme authors to style Chromium widgets specially

chrome/browser/ui/libgtkui/gtk_util.cc

Index: chrome/browser/ui/libgtkui/gtk_util.cc
diff --git a/chrome/browser/ui/libgtkui/gtk_util.cc b/chrome/browser/ui/libgtkui/gtk_util.cc
index 81fefa61d5249e56a8007adbb555deca41fcf528..633539dfac49f211e723146fe46f184fee9a759c 100644
--- a/chrome/browser/ui/libgtkui/gtk_util.cc
+++ b/chrome/browser/ui/libgtkui/gtk_util.cc
@@ -294,6 +294,11 @@ ScopedStyleContext AppendNode(GtkStyleContext* context,
       }
     }
   }
+
+  // Always add a "chromium" class so that themes can style chromium
+  // widgets specially if they want to.
+  gtk_widget_path_iter_add_class(path, -1, "chromium");
+
   auto child_context = ScopedStyleContext(gtk_style_context_new());
   gtk_style_context_set_path(child_context, path);
   gtk_style_context_set_state(child_context, state);