Gtk3: Render a GtkHeaderBar as the background of the tab strip

ui/native_theme/native_theme_base.h

Index: ui/native_theme/native_theme_base.h
diff --git a/ui/native_theme/native_theme_base.h b/ui/native_theme/native_theme_base.h
index f47129381cdccca194daad59841cb7d2bf12029d..fa06cfbceb78dbdfb7f6cb332f3519207c61c64b 100644
--- a/ui/native_theme/native_theme_base.h
+++ b/ui/native_theme/native_theme_base.h
@@ -128,6 +128,12 @@ class NATIVE_THEME_EXPORT NativeThemeBase : public NativeTheme {
       const gfx::Rect& rect,
       const ProgressBarExtraParams& progress_bar) const;
 
+  virtual void PaintFrameTopArea(
+      SkCanvas* canvas,
+      State state,
+      const gfx::Rect& rect,
+      const FrameTopAreaExtraParams& frame_top_area) const;
+
   // Shrinks checkbox/radio button rect, if necessary, to make room for padding
   // and drop shadow.
   // TODO(mohsen): This is needed because checkboxes/radio buttons on Android