Put STHSet component behind a base::Feature.

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index 54df3f5641ce39e4442445d9eab7d90651824854..d4c9ab27245a5f0d51410cba61bf12c897b8d2a0 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -497,10 +497,12 @@ void RegisterComponentsForUpdate() {
     // 2. Chrome OS: On Chrome OS this registration is delayed until user login.
     RegisterEVWhitelistComponent(cus, path);
 
-    // Registration of the STH set fetcher here is not done for:
-    // Android: Because the story around CT on Mobile is not finalized yet.
-    // Chrome OS: On Chrome OS this registration is delayed until user login.
-    RegisterSTHSetComponent(cus, path);
+    if (base::FeatureList::IsEnabled(features::kSTHSetComponent)) {

[Eran Messeri, 2016/07/08 15:39:36]

Please document the experiment / link to a bug.

[Alexei Svitkine (slow), 2016/07/08 15:42:15]

Done.

+      // Registration of the STH set fetcher here is not done for:
+      // Android: Because the story around CT on Mobile is not finalized yet.
+      // Chrome OS: On Chrome OS this registration is delayed until user login.
+      RegisterSTHSetComponent(cus, path);
+    }
 #endif  // defined(OS_ANDROID)
     RegisterOriginTrialsComponent(cus, path);