Mac Dark Mode: fix menu separators

Bug: 850098
Change-Id: I174a01a66f7b8c0008f24a676f32b11347c33660
Reviewed-on: https://chromium-review.googlesource.com/c/1391956Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619407}

ui/native_theme/native_theme_mac.mm

@@ -190,7 +190,8 @@ SkColor NativeThemeMac::GetSystemColor(ColorId color_id) const {
     case kColorId_DisabledMenuItemForegroundColor:
       return NSSystemColorToSkColor([NSColor disabledControlTextColor]);
     case kColorId_MenuSeparatorColor:
-      return SkColorSetA(SK_ColorBLACK, 0x26);
+      return SystemDarkModeEnabled() ? SkColorSetA(gfx::kGoogleGrey800, 0xCC)
+                                     : SkColorSetA(SK_ColorBLACK, 0x26);
     case kColorId_MenuBorderColor:
       return SkColorSetA(SK_ColorBLACK, 0x60);