Add a new GetSystemColor method to native theme.

ui/gfx/native_theme_base.cc

Index: ui/gfx/native_theme_base.cc
diff --git a/ui/gfx/native_theme_base.cc b/ui/gfx/native_theme_base.cc
index 95ba5aa1e681b1856e4f246b1c6837bab2157c68..22787ac0731ed1de80a4a29f8df3a5a5d1d83866 100644
--- a/ui/gfx/native_theme_base.cc
+++ b/ui/gfx/native_theme_base.cc
@@ -14,6 +14,14 @@
 #include "ui/gfx/rect.h"
 #include "ui/gfx/size.h"
 
+namespace {
+
+// Hardcoded colors for use when there is no system theme (Aura, ChromeOS).
+const SkColor kDefaultDialogBackgroundColor = SkColorSetRGB(200, 200, 200);
+const SkColor kInvalidColorIdColor = SkColorSetRGB(255, 0, 128);
+
+}  // namespace
+
 namespace gfx {
 
 unsigned int NativeThemeBase::button_length_ = 14;
@@ -334,6 +342,19 @@ void NativeThemeBase::Paint(SkCanvas* canvas,
   }
 }
 
+SkColor NativeThemeBase::GetSystemColor(ColorId color_id) const {
+  // This implementation returns hardcoded colors. It's used by NativeThemeAura
+  // and NativeThemeChromeos and overridden by NativeThemeGtk.
+  switch (color_id) {
+    case kColorId_DialogBackground:
+      return kDefaultDialogBackgroundColor;
+    default:
+      NOTREACHED() << "Invalid color_id: " << color_id;
+      break;
+  }
+  return kInvalidColorIdColor;
+}
+
 void NativeThemeBase::PaintScrollbarTrack(SkCanvas* canvas,
     Part part,
     State state,