Update to Chromium version 68.0.3425.0 (#557062)

Known issues:
- Surface synchronization is now enabled for OSR, see issue #2447.
- Jumbo build is broken, fixed in https://crrev.com/868717ce.

libcef/browser/chrome_browser_process_stub.cc

@@ -346,12 +346,6 @@ resource_coordinator::TabManager* ChromeBrowserProcessStub::GetTabManager() {
   return NULL;
 }
 
-physical_web::PhysicalWebDataSource*
-ChromeBrowserProcessStub::GetPhysicalWebDataSource() {
-  NOTREACHED();
-  return NULL;
-}
-
 prefs::InProcessPrefServiceFactory*
 ChromeBrowserProcessStub::pref_service_factory() const {
   NOTREACHED();