Have Chrome draw top window border when using custom titlebar.

chrome/browser/themes/theme_service_win.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_THEMES_THEME_SERVICE_WIN_H_
 #define CHROME_BROWSER_THEMES_THEME_SERVICE_WIN_H_
 
 #include "base/win/registry.h"
 #include "chrome/browser/themes/theme_service.h"
 
@@ skipping 15 matching lines @@
   // frame color and updates |dwm_frame_color_|.
   void OnDwmKeyUpdated();
 
   // Registry key containing the params that determine the DWM frame color.
   // This is only initialized on Windows 10.
   std::unique_ptr<base::win::RegKey> dwm_key_;
 
   // The DWM frame color, if available; white otherwise.
   SkColor dwm_frame_color_;
 
+  // The DWM accent border color, if available; white otherwise.
+  SkColor dwm_accent_color_;

[Bret, 2016/10/06 20:48:09]

Hmm this name is confusing because the AccentColor is actually the frame color,
even if it shows up in more limited circumstances than the border color. Maybe
just call it dwm_border_color_? Or maybe dwm_accent_border_color_ which is
consistent with what you're calling it everywhere else.

+
   DISALLOW_COPY_AND_ASSIGN(ThemeServiceWin);
 };
 
 #endif  // CHROME_BROWSER_THEMES_THEME_SERVICE_WIN_H_