WebUI Dark Mode: enable by default on Mac

R=thestig@chromium.org
BUG=883049

Change-Id: I2dc81de46b816debcb8e31042c4078e91331e258
Reviewed-on: https://chromium-review.googlesource.com/c/1492977
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636285}

chrome/browser/browser_features.cc

@@ -17,8 +17,12 @@ const base::Feature kDoubleTapToZoomInTabletMode{
 // Whether to enable "dark mode" enhancements in Mac Mojave or Windows 10 for
 // UIs implemented with web technologies.
 const base::Feature kWebUIDarkMode{"WebUIDarkMode",
+#if defined(OS_MACOSX)
+                                   base::FEATURE_ENABLED_BY_DEFAULT};
+#else
                                    base::FEATURE_DISABLED_BY_DEFAULT};
-#endif
+#endif  // defined(OS_MACOSX)
+#endif  // !defined(OS_ANDROID)
 
 // Enables grouping tabs together in the tab strip. https://crbug.com/905491
 const base::Feature kTabGroups{"TabGroups", base::FEATURE_DISABLED_BY_DEFAULT};