Introduce flag to disable static popular sites.

components/ntp_tiles/constants.h

Index: components/ntp_tiles/constants.h
diff --git a/components/ntp_tiles/constants.h b/components/ntp_tiles/constants.h
index 8c50a99d7119e902255cbbec20dff66040755975..42651086328de2bd31504d6b537736f69fefb4f6 100644
--- a/components/ntp_tiles/constants.h
+++ b/components/ntp_tiles/constants.h
@@ -5,11 +5,20 @@
 #ifndef COMPONENTS_NTP_TILES_CONSTANTS_H_
 #define COMPONENTS_NTP_TILES_CONSTANTS_H_
 
+namespace base {
+struct Feature;
+}  // namespace base
+
 namespace ntp_tiles {
 
 // Name of the field trial to configure PopularSites.
 extern const char kPopularSitesFieldTrialName[];
 
+// This feature is enabled by default. Otherwise, users who need it would not
+// get the right configuration timely enough. The configuration affects only
+// Android or iOS users.
+extern const base::Feature kPopularSitesBakedInContentFeature;
+
 }  // namespace ntp_tiles
 
-#endif
+#endif  // COMPONENTS_NTP_TILES_CONSTANTS_H_