Gtk3: Add background class to tooltips

chrome/browser/ui/libgtkui/gtk_util.h

Index: chrome/browser/ui/libgtkui/gtk_util.h
diff --git a/chrome/browser/ui/libgtkui/gtk_util.h b/chrome/browser/ui/libgtkui/gtk_util.h
index ab335cdb0d6857283f2fc0ca8a2d5dcd18a92ec8..cc6725b7463eef7522a86e4d584c2ab673e06233 100644
--- a/chrome/browser/ui/libgtkui/gtk_util.h
+++ b/chrome/browser/ui/libgtkui/gtk_util.h
@@ -166,6 +166,8 @@ ScopedStyleContext GetStyleContextFromCss(const std::string& css_selector);
 
 SkColor GetFgColorFromStyleContext(GtkStyleContext* context);
 
+SkColor GetBgColorFromStyleContext(GtkStyleContext* context);
+
 // Overrides properties on |context| and all its parents with those
 // provided by |css|.
 void ApplyCssToContext(GtkStyleContext* context, const std::string& css);