Put STHSet component behind a base::Feature.

chrome/common/chrome_features.cc

Index: chrome/common/chrome_features.cc
diff --git a/chrome/common/chrome_features.cc b/chrome/common/chrome_features.cc
index f9134ad844f5c9bb241a9f52977269142d44b640..761c61313796b670a207a50ea74c31e536c41abf 100644
--- a/chrome/common/chrome_features.cc
+++ b/chrome/common/chrome_features.cc
@@ -78,6 +78,10 @@ const base::Feature kSafeSearchUrlReporting{"SafeSearchUrlReporting",
 const base::Feature kSimplifiedFullscreenUI{"ViewsSimplifiedFullscreenUI",
                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
+// Enables or disables the STH component-updater component.
+const base::Feature kSTHSetComponent{"STHSetComponent",
+                                     base::FEATURE_ENABLED_BY_DEFAULT};
+
 #if defined(SYZYASAN)
 // Enable the deferred free mechanism in the syzyasan module, which helps the
 // performance by deferring some work on the critical path to a background