[iOS] Upstream some stability code

ios/public/provider/chrome/browser/chrome_browser_provider.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 
 #include "base/logging.h"
 
 namespace ios {
 
@@ skipping 88 matching lines @@
 std::string ChromeBrowserProvider::GetProductVersionWithPrefix(
     const std::string& prefix) {
   NOTREACHED();
   return std::string();
 }
 
 std::string ChromeBrowserProvider::GetVersionString() {
   return std::string();
 }
 
+std::string ChromeBrowserProvider::GetVersionNumber() {
+  return std::string();
+}
+
 }  // namespace ios