Have Chrome draw top window border when using custom titlebar.

chrome/browser/themes/theme_service_win.cc

Index: chrome/browser/themes/theme_service_win.cc
diff --git a/chrome/browser/themes/theme_service_win.cc b/chrome/browser/themes/theme_service_win.cc
index 4aab2c4219d3244790e283ae8a1a8e88eccb7f66..55497dab941ba7d9dc0ee548eeaf8f1e8a2f0eca 100644
--- a/chrome/browser/themes/theme_service_win.cc
+++ b/chrome/browser/themes/theme_service_win.cc
@@ -10,6 +10,8 @@
 #include "chrome/grit/theme_resources.h"
 #include "skia/ext/skia_utils_win.h"
 #include "ui/base/win/shell.h"
+#include "ui/gfx/color_utils.h"
+#include "ui/gfx/geometry/safe_integer_conversions.h"
 
 ThemeServiceWin::ThemeServiceWin() {
   // This just checks for Windows 10 instead of calling ShouldUseDwmFrameColor()
@@ -39,6 +41,9 @@ SkColor ThemeServiceWin::GetDefaultColor(int id, bool incognito) const {
     if (id == ThemeProperties::COLOR_FRAME)
       return dwm_frame_color_;
 
+    if (id == ThemeProperties::COLOR_ACCENT_BORDER)
+      return dwm_accent_border_color_;
+
     // Inactive native windows on Windows 10 always have a white frame.
     if (id == ThemeProperties::COLOR_FRAME_INACTIVE)
         return SK_ColorWHITE;
@@ -63,6 +68,33 @@ void ThemeServiceWin::OnDwmKeyUpdated() {
           ? skia::COLORREFToSkColor(accent_color)
           : SK_ColorWHITE;
 
+  dwm_accent_border_color_ = SK_ColorWHITE;
+  DWORD colorization_color, colorization_color_balance;
+  if ((dwm_key_->ReadValueDW(L"ColorizationColor", &colorization_color) ==
+       ERROR_SUCCESS) &&
+      (dwm_key_->ReadValueDW(L"ColorizationColorBalance",
+                             &colorization_color_balance) == ERROR_SUCCESS)) {
+    // The accent border color is a linear blend between the colorization
+    // color and the neutral #d9d9d9. colorization_color_balance is the
+    // percentage of the colorization color in that blend.
+    //
+    // On Windows version 1611 colorization_color_balance can be 0xfffffff3 if
+    // the accent color is taken from the background and either the background
+    // is a solid color or was just changed to a slideshow. It's unclear what
+    // that value's supposed to mean, so change it to 80 to match Edge's
+    // behavior.
+    if (colorization_color_balance > 100)
+      colorization_color_balance = 80;
+
+    // colorization_color's high byte is not an alpha value, so replace it
+    // with 0xff to make an opaque ARGB color.
+    SkColor input_color = SkColorSetA(colorization_color, 0xff);
+
+    dwm_accent_border_color_ = color_utils::AlphaBlend(
+        input_color, SkColorSetRGB(0xd9, 0xd9, 0xd9),
+        gfx::ToRoundedInt(255 * colorization_color_balance / 100.f));
+  }
+
   // Watch for future changes.
   if (!dwm_key_->StartWatching(base::Bind(
           &ThemeServiceWin::OnDwmKeyUpdated, base::Unretained(this))))