cocoa: draw BackgroundGradientView separators darker in Increase Contrast mode

chrome/browser/ui/cocoa/background_gradient_view.mm

Index: chrome/browser/ui/cocoa/background_gradient_view.mm
diff --git a/chrome/browser/ui/cocoa/background_gradient_view.mm b/chrome/browser/ui/cocoa/background_gradient_view.mm
index b56f3d0551037bff7704194803f7d524c6c4303e..aa16f6d9ef9ada606045dee95842b0a5e4022290 100644
--- a/chrome/browser/ui/cocoa/background_gradient_view.mm
+++ b/chrome/browser/ui/cocoa/background_gradient_view.mm
@@ -98,6 +98,12 @@ - (NSColor*)strokeColor {
   const ui::ThemeProvider* themeProvider = [window themeProvider];
   if (!themeProvider)
     return [NSColor blackColor];
+  if (themeProvider->ShouldIncreaseContrast()) {
+    if ([window hasDarkTheme])
+      return [NSColor whiteColor];
+    else
+      return [NSColor blackColor];
+  }
   return themeProvider->GetNSColor(
              ThemeProperties::COLOR_DETACHED_BOOKMARK_BAR_SEPARATOR);
 }