Dark Mode: add a base::Feature to respond to dark mode in WebUI

Example combined usage CL: https://crrev.com/c/1354685

BUG=883049
R=thestig@chromium.org

Change-Id: Ie6f44e2095a0c59738acec58b478b5a5e22bbd2f
Reviewed-on: https://chromium-review.googlesource.com/c/1357627Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612896}

chrome/browser/browser_features.cc

@@ -13,4 +13,11 @@ const base::Feature kDoubleTapToZoomInTabletMode{
     "DoubleTapToZoomInTabletMode", base::FEATURE_DISABLED_BY_DEFAULT};
 #endif
 
+#if !defined(OS_ANDROID)
+// Whether to enable "dark mode" enhancements in Mac Mojave or Windows 10 for
+// UIs implemented with web technologies.
+const base::Feature kWebUIDarkMode{"WebUIDarkMode",
+                                   base::FEATURE_DISABLED_BY_DEFAULT};
+#endif
+
 }  // namespace features

chrome/browser/browser_features.h

@@ -9,6 +9,7 @@
 #define CHROME_BROWSER_BROWSER_FEATURES_H_
 
 #include "base/feature_list.h"
+#include "build/build_config.h"
 
 namespace features {
 
@@ -19,6 +20,10 @@ namespace features {
 extern const base::Feature kDoubleTapToZoomInTabletMode;
 #endif
 
+#if !defined(OS_ANDROID)
+extern const base::Feature kWebUIDarkMode;
+#endif
+
 }  // namespace features
 
 #endif  // CHROME_BROWSER_BROWSER_FEATURES_H_