Don't delay creation of system URLRequestContext until first use

components/proxy_config/pref_proxy_config_tracker.h

Index: components/proxy_config/pref_proxy_config_tracker.h
diff --git a/components/proxy_config/pref_proxy_config_tracker.h b/components/proxy_config/pref_proxy_config_tracker.h
index d51abc8ae947b706875ba481b8fefd3331817a11..77e2e36ccfe50c049ede3f562482223f57e840d0 100644
--- a/components/proxy_config/pref_proxy_config_tracker.h
+++ b/components/proxy_config/pref_proxy_config_tracker.h
@@ -29,7 +29,7 @@ class PROXY_CONFIG_EXPORT PrefProxyConfigTracker {
   // 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;