clang/win: Let some chromium_code targets build with -Wextra.

ui/native_theme/native_theme_win.cc

Index: ui/native_theme/native_theme_win.cc
diff --git a/ui/native_theme/native_theme_win.cc b/ui/native_theme/native_theme_win.cc
index cf05c89fc34212332360b5ce99b207663747c39d..91834768ca11e4528cb6a60c72c943951c4a227c 100644
--- a/ui/native_theme/native_theme_win.cc
+++ b/ui/native_theme/native_theme_win.cc
@@ -356,10 +356,8 @@ void NativeThemeWin::OnSysColorChange() {
 }
 
 void NativeThemeWin::UpdateSystemColors() {
-  for (int i = 0; i < arraysize(kSystemColors); ++i) {
-    system_colors_[kSystemColors[i]] =
-        color_utils::GetSysSkColor(kSystemColors[i]);
-  }
+  for (int kSystemColor : kSystemColors)
+    system_colors_[kSystemColor] = color_utils::GetSysSkColor(kSystemColor);
 }
 
 void NativeThemeWin::PaintDirect(SkCanvas* canvas,