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

ui/native_theme/native_theme_base.cc

Index: ui/native_theme/native_theme_base.cc
diff --git a/ui/native_theme/native_theme_base.cc b/ui/native_theme/native_theme_base.cc
index b9d2827db8961cd0c240bcf7c01f23fde6359027..ddf2128cbd446991458e4fe61cf35b84b6de1161 100644
--- a/ui/native_theme/native_theme_base.cc
+++ b/ui/native_theme/native_theme_base.cc
@@ -172,6 +172,9 @@ void NativeThemeBase::Paint(SkCanvas* canvas,
     case kCheckbox:
       PaintCheckbox(canvas, state, rect, extra.button);
       break;
+    case kFrameTopArea:
+      PaintFrameTopArea(canvas, state, rect, extra.frame_top_area);
+      break;
     case kInnerSpinButton:
       PaintInnerSpinButton(canvas, state, rect, extra.inner_spin);
       break;
@@ -898,6 +901,16 @@ void NativeThemeBase::PaintProgressBar(
   canvas->drawRect(gfx::RectFToSkRect(border_rect), stroke_paint);
 }
 
+void NativeThemeBase::PaintFrameTopArea(
+    SkCanvas* canvas,
+    State state,
+    const gfx::Rect& rect,
+    const FrameTopAreaExtraParams& frame_top_area) const {
+  SkPaint paint;
+  paint.setColor(frame_top_area.default_background_color);
+  canvas->drawRect(gfx::RectToSkRect(rect), paint);
+}
+
 void NativeThemeBase::AdjustCheckboxRadioRectForPadding(SkRect* rect) const {
   // By default we only take 1px from right and bottom for the drop shadow.
   rect->iset(rect->x(), rect->y(), rect->right() - 1, rect->bottom() - 1);