shill: Convert ProxyFactory singleton to LazyInstance.

This makes it consistent with the rest of shill's singletons (DHCPProvider,
Resolver, RoutingTable, RTNLHandler) and improves static object's memory
management.

BUG=chromium-os:19178
TEST=unit tests, tested in a VM

Change-Id: Ib403d31473360b46dd25d12508c799cbc73368fd
Reviewed-on: http://gerrit.chromium.org/gerrit/8861
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/dhcp_config_unittest.cc b/dhcp_config_unittest.cc
index d6d3929..a8b4512 100644
--- a/dhcp_config_unittest.cc
+++ b/dhcp_config_unittest.cc
@@ -44,11 +44,11 @@
                                glib())) {}
 
   virtual void SetUp() {
-    ProxyFactory::set_factory(&proxy_factory_);
+    config_->proxy_factory_ = &proxy_factory_;
   }
 
   virtual void TearDown() {
-    ProxyFactory::set_factory(NULL);
+    config_->proxy_factory_ = NULL;
   }
 
  protected: