Add feature flag to allow dark mode if enabled on system or forced

Bug: 850098
Change-Id: Ib2af48ceb226bbe2650c65331d4bbec2321c0956
Reviewed-on: https://chromium-review.googlesource.com/1251328Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595113}

ui/base/ui_base_features.cc

@@ -171,4 +171,6 @@ bool IsOzoneDrmMojo() {
          IsMultiProcessMash();
 }
 
+const base::Feature kDarkMode = {"DarkMode", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features

ui/base/ui_base_features.h

@@ -90,6 +90,10 @@ UI_BASE_EXPORT bool IsViewsBrowserCocoa();
 UI_BASE_EXPORT extern const base::Feature kEnableOzoneDrmMojo;
 UI_BASE_EXPORT bool IsOzoneDrmMojo();
 
+// Whether default UI should use a dark mode color scheme, if enabled on
+// macOS Mojave/Windows 10, or the --force-dark-mode flag is provided.
+UI_BASE_EXPORT extern const base::Feature kDarkMode;
+
 }  // namespace features
 
 #endif  // UI_BASE_UI_BASE_FEATURES_H_