Enable Dark Mode by default on Mac

Bug: 850098
Change-Id: I454c77d5c47fcdc90ed7a4265cf4f268186a983b
Reviewed-on: https://chromium-review.googlesource.com/c/1431715
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625671}
diff --git a/ui/base/ui_base_features.cc b/ui/base/ui_base_features.cc
index 71bd2fb..07f73f8 100644
--- a/ui/base/ui_base_features.cc
+++ b/ui/base/ui_base_features.cc
@@ -176,7 +176,11 @@
          IsMultiProcessMash();
 }
 
+#if defined(OS_MACOSX)
+const base::Feature kDarkMode = {"DarkMode", base::FEATURE_ENABLED_BY_DEFAULT};
+#else
 const base::Feature kDarkMode = {"DarkMode", base::FEATURE_DISABLED_BY_DEFAULT};
+#endif
 
 #if defined(OS_CHROMEOS)
 const base::Feature kHandwritingGesture = {"HandwritingGesture",