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..1cfe9fd6f1e82a522b6d3a47107d0993a4fae236 100644
--- a/ui/gfx/native_theme_base.cc
+++ b/ui/gfx/native_theme_base.cc
@@ -16,6 +16,11 @@
 
 namespace gfx {
 
+namespace {
+// Hardcoded colors for use when there is no system theme (Aura, ChromeOS).
+const SkColor kDefaultDialogBackgroundColor = SkColorSetRGB(200, 200, 200);
+}
+
 unsigned int NativeThemeBase::button_length_ = 14;
 #if defined(TOUCH_UI)
 unsigned int NativeThemeBase::scrollbar_width_ = 0;
@@ -57,6 +62,19 @@ NativeThemeBase::NativeThemeBase() {
 NativeThemeBase::~NativeThemeBase() {
 }
 
+// This implementation returns hardcoded colors. It's used by NativeThemeAura
+// and NativeThemeChromeos and overridden by NativeThemeGtk.
+SkColor NativeThemeBase::GetSystemColor(ColorId color_id) const {
+  switch (color_id) {
+    case kDialogBackgroundColorId:
+      return kDefaultDialogBackgroundColor;
+    default:
+      NOTREACHED() << "Invalid color_id: " << color_id;
+      break;
+  }
+  return kColorYouShouldNeverSee;
+}
+
 gfx::Size NativeThemeBase::GetPartSize(Part part,
                                        State state,
                                        const ExtraParams& extra) const {