Gtk3: Allow theme authors to style Chromium widgets specially

chrome/browser/ui/libgtkui/gtk_util.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/libgtkui/gtk_util.h"
 
 #include <dlfcn.h>
 #include <gdk/gdk.h>
 #include <gdk/gdkx.h>
 #include <gtk/gtk.h>
@@ skipping 276 matching lines @@
               state_flag = pseudo_class_entry.state_flag;
               break;
             }
           }
           state = static_cast<GtkStateFlags>(state | state_flag);
           break;
         }
       }
     }
   }
+
+  // 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);
   gtk_style_context_set_parent(child_context, context);
   gtk_widget_path_unref(path);
   return child_context;
 }
 
 ScopedStyleContext GetStyleContextFromCss(const char* css_selector) {
   // Prepend "GtkWindow.background" to the selector since all widgets must live
@@ skipping 126 matching lines @@
 
   AddBorders(context);
   PixelSurface surface;
   RenderBackground(surface.cairo(), context);
   gtk_render_frame(context, surface.cairo(), 0, 0, 1, 1);
   return surface.GetPixelValue();
 }
 #endif
 
 }  // namespace libgtkui