Update to Chromium version 70.0.3538.0 (#587811)

Known issues:
- The CefLoadCRLSetsFile function needs to be re-implemented (see issue #2497).
- Linux: GTK2 support has been removed. The cefclient sample needs to be updated
  to use GTK3 (see issue #2014).

libcef/browser/chrome_browser_process_stub.cc

@@ -111,12 +111,6 @@ ChromeBrowserProcessStub::system_network_context_manager() {
   return NULL;
 }
 
-network::NetworkConnectionTracker*
-ChromeBrowserProcessStub::network_connection_tracker() {
-  NOTREACHED();
-  return NULL;
-}
-
 network::NetworkQualityTracker*
 ChromeBrowserProcessStub::network_quality_tracker() {
   NOTREACHED();