[iOS] Upstream switches and flags controlling which WebView is used

ios/chrome/browser/experimental_flags.h

 // Copyright 2014 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.
 
 #ifndef IOS_CHROME_BROWSER_EXPERIMENTAL_FLAGS_H_
 #define IOS_CHROME_BROWSER_EXPERIMENTAL_FLAGS_H_
 
 #include <stdlib.h>
 
 // This file can be empty. Its purpose is to contain the relatively short lived
 // declarations required for experimental flags.
 
 namespace experimental_flags {
 
 // Whether background crash report upload should generate a local notification.
 bool IsAlertOnBackgroundUploadEnabled();
 
 // Returns true if the contents of the clipboard can be used for autocomplete.
 bool IsOpenFromClipboardEnabled();
 
+// Whether the app uses WKWebView instead of UIWebView.
+bool IsWKWebViewEnabled();
+
 // Returns the size in MB of the memory wedge to insert during a cold start.
 // If 0, no memory wedge should be inserted.
 size_t MemoryWedgeSizeInMB();
 
 }  // namespace experimental_flags
 
 #endif  // IOS_CHROME_BROWSER_EXPERIMENTAL_FLAGS_H_