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

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 5f7a9d9ba5d82d55801f647f807d92fc3a029a87..d25806ca42fe844f90731e3dc18fcf0a8171a7d4 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -45,6 +45,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
   // The part to be painted / sized.
   enum Part {
     kCheckbox,
+    kFrameTopArea,
     kInnerSpinButton,
     kMenuList,
     kMenuPopupBackground,
@@ -109,6 +110,11 @@ class NATIVE_THEME_EXPORT NativeTheme {
     SkColor background_color;
   };
 
+  struct FrameTopAreaExtraParams {
+    bool is_active;
+    SkColor default_background_color;  // The theme can ignore this color.
+  };
+
   struct InnerSpinButtonExtraParams {
     bool spin_up;
     bool read_only;
@@ -207,6 +213,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
     ExtraParams(const ExtraParams& other);
 
     ButtonExtraParams button;
+    FrameTopAreaExtraParams frame_top_area;
     InnerSpinButtonExtraParams inner_spin;
     MenuArrowExtraParams menu_arrow;
     MenuCheckExtraParams menu_check;