Clean up some NativeTheme code.

ui/native_theme/native_theme_android.cc

Index: ui/native_theme/native_theme_android.cc
diff --git a/ui/native_theme/native_theme_android.cc b/ui/native_theme/native_theme_android.cc
index 7f799d9a1abaa0574d650a605829afaddec52a29..cd6bc1d04aa29aa16419d6b4b0d68b357149b0bb 100644
--- a/ui/native_theme/native_theme_android.cc
+++ b/ui/native_theme/native_theme_android.cc
@@ -20,6 +20,11 @@ const int kCheckboxAndRadioHeight = 16;
 NativeTheme* NativeTheme::GetInstanceForWeb() {
   return NativeThemeAndroid::instance();
 }
+
+NativeTheme* NativeTheme::GetInstanceForNativeUi() {
+  NOTREACHED();
+  return nullptr;
+}
 #endif
 
 // static