Put STHSet component behind a base::Feature.

chrome/browser/chrome_browser_main.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chrome_browser_main.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
@@ skipping 479 matching lines @@
 #elif !defined(OS_CHROMEOS)
     // CRLSetFetcher attempts to load a CRL set from either the local disk or
     // network.
     // For Chrome OS this registration is delayed until user login.
     g_browser_process->crl_set_fetcher()->StartInitialLoad(cus, path);
     // Registration of the EV Whitelist component here is not necessary for:
     // 1. Android: Because it currently does not have the EV indicator.
     // 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);
 
     RegisterFileTypePoliciesComponent(cus, path);
   }
 
 #if defined(OS_WIN)
 #if defined(GOOGLE_CHROME_BUILD)
   RegisterSwReporterComponent(cus);
 #endif  // defined(GOOGLE_CHROME_BUILD)
@@ skipping 1520 matching lines @@
   chromeos::CrosSettings::Shutdown();
 #endif  // defined(OS_CHROMEOS)
 #endif  // defined(OS_ANDROID)
 }
 
 // Public members:
 
 void ChromeBrowserMainParts::AddParts(ChromeBrowserMainExtraParts* parts) {
   chrome_extra_parts_.push_back(parts);
 }