Add initial NetworkService support (see issue #2622).

This change adds an --enable-network-service command-line flag to run with
NetworkService enabled.

To test: Run `cefclient --enable-network-service --disable-extensions`. The
application should start, load a website and exit without crashing.
Network-related handlers (for cookies, schemes, requests, etc) and extensions
are not expected to work yet.

There should be no functional change when running without the NetworkService
enabled.

libcef/browser/chrome_browser_process_stub.h

@@ -126,6 +126,10 @@ class ChromeBrowserProcessStub : public BrowserProcess,
   scoped_refptr<extensions::EventRouterForwarder> event_router_forwarder_;
   std::unique_ptr<net_log::ChromeNetLog> net_log_;
   std::unique_ptr<net_log::NetExportFileWriter> net_export_file_writer_;
+  std::unique_ptr<PrefService> local_state_;
+  // Must be destroyed after |local_state_|.
+  std::unique_ptr<policy::ChromeBrowserPolicyConnector>
+      browser_policy_connector_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeBrowserProcessStub);
 };