Don't delay creation of system URLRequestContext until first use

components/proxy_config/pref_proxy_config_tracker.h

 // Copyright (c) 2011 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.
 
 #ifndef COMPONENTS_PROXY_CONFIG_PREF_PROXY_CONFIG_TRACKER_H_
 #define COMPONENTS_PROXY_CONFIG_PREF_PROXY_CONFIG_TRACKER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 11 matching lines @@
  public:
   PrefProxyConfigTracker();
   virtual ~PrefProxyConfigTracker();
 
   // Creates a net::ProxyConfigService and keeps a pointer to it. After this
   // call, this tracker forwards any changes of proxy preferences to the created
   // ProxyConfigService. The returned ProxyConfigService must not be deleted
   // before DetachFromPrefService was called. Takes ownership of the passed
   // |base_service|, which can be NULL. This |base_service| provides the proxy
   // settings of the OS (except of ChromeOS). This must be called on the
-  // UI thread.
+  // UI thread.  May only be called once on a PrefProxyConfigTracker.
   virtual std::unique_ptr<net::ProxyConfigService>
   CreateTrackingProxyConfigService(
       std::unique_ptr<net::ProxyConfigService> base_service) = 0;
 
   // Releases the PrefService passed upon construction and the |base_service|
   // passed to CreateTrackingProxyConfigService. This must be called on the UI
   // thread.
   virtual void DetachFromPrefService() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PrefProxyConfigTracker);
 };
 
 #endif  // COMPONENTS_PROXY_CONFIG_PREF_PROXY_CONFIG_TRACKER_H_