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

ios/chrome/browser/experimental_flags.h

Index: ios/chrome/browser/experimental_flags.h
diff --git a/ios/chrome/browser/experimental_flags.h b/ios/chrome/browser/experimental_flags.h
index 597641800190278ed4e8f6ed216eaead7d3b1ab9..87839f0e034c1bff35fe0c95b825f41480b7c653 100644
--- a/ios/chrome/browser/experimental_flags.h
+++ b/ios/chrome/browser/experimental_flags.h
@@ -18,6 +18,9 @@ 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();