Introduce flag to disable static popular sites.

components/ntp_tiles/constants.cc

Index: components/ntp_tiles/constants.cc
diff --git a/components/ntp_tiles/constants.cc b/components/ntp_tiles/constants.cc
index 7e3d84f9de1398164b40a2015429e9130865ce5f..0c54388e9a8a95371d1c12d2ff9740664f5495dc 100644
--- a/components/ntp_tiles/constants.cc
+++ b/components/ntp_tiles/constants.cc
@@ -4,8 +4,13 @@
 
 #include "components/ntp_tiles/constants.h"
 
+#include "base/feature_list.h"
+
 namespace ntp_tiles {
 
 const char kPopularSitesFieldTrialName[] = "NTPPopularSites";
 
+extern const base::Feature kPopularSitesBakedInContentFeature{
+    "NTPPopularSitesBakedInContent", base::FEATURE_ENABLED_BY_DEFAULT};
+
 }  // namespace ntp_tiles