Introduce a type of View background that stays in sync with its host

ui/views/background.cc

Index: ui/views/background.cc
diff --git a/ui/views/background.cc b/ui/views/background.cc
index 190a5a62c346e714f9b7c4dad8a8d79321132a0d..e036490a0c24950c931077dd106f41cbbad5b055 100644
--- a/ui/views/background.cc
+++ b/ui/views/background.cc
@@ -6,11 +6,14 @@
 
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/scoped_observer.h"
 #include "build/build_config.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/color_palette.h"
 #include "ui/gfx/color_utils.h"
 #include "ui/views/painter.h"
 #include "ui/views/view.h"
+#include "ui/views/view_observer.h"
 
 #if defined(OS_WIN)
 #include "skia/ext/skia_utils_win.h"
@@ -36,6 +39,32 @@ class SolidBackground : public Background {
   DISALLOW_COPY_AND_ASSIGN(SolidBackground);
 };
 
+// ThemedSolidBackground is a solid background that stays in sync with a view's
+// native theme.
+class ThemedSolidBackground : public SolidBackground, public ViewObserver {
+ public:
+  explicit ThemedSolidBackground(View* view, ui::NativeTheme::ColorId color_id)
+      : SolidBackground(gfx::kPlaceholderColor),
+        observer_(this),
+        color_id_(color_id) {
+    observer_.Add(view);
+    OnViewNativeThemeChanged(view);
+  }
+  ~ThemedSolidBackground() override {}
+
+  // ViewObserver:
+  void OnViewNativeThemeChanged(View* view) override {
+    SetNativeControlColor(view->GetNativeTheme()->GetSystemColor(color_id_));
+    view->SchedulePaint();
+  }
+
+ private:
+  ScopedObserver<View, ViewObserver> observer_;
+  ui::NativeTheme::ColorId color_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThemedSolidBackground);
+};
+
 class BackgroundPainter : public Background {
  public:
   explicit BackgroundPainter(std::unique_ptr<Painter> painter)
@@ -73,6 +102,13 @@ Background* Background::CreateSolidBackground(SkColor color) {
 }
 
 // static
+Background* Background::CreateThemedSolidBackground(
+    View* view,
+    ui::NativeTheme::ColorId color_id) {
+  return new ThemedSolidBackground(view, color_id);
+}
+
+// static
 Background* Background::CreateStandardPanelBackground() {
   // TODO(beng): Should be in NativeTheme.
   return CreateSolidBackground(SK_ColorWHITE);