[Navigation Experiment] Add WKBasedNavigationManagerImpl.

ios/web/web_state/web_state_impl.mm

Index: ios/web/web_state/web_state_impl.mm
diff --git a/ios/web/web_state/web_state_impl.mm b/ios/web/web_state/web_state_impl.mm
index d918d551104742002c9fe8f0f0c4464aceed938f..c7511f020ada2a2c223233029ae29576c20477f4 100644
--- a/ios/web/web_state/web_state_impl.mm
+++ b/ios/web/web_state/web_state_impl.mm
@@ -18,6 +18,9 @@
 #import "ios/web/navigation/legacy_navigation_manager_impl.h"
 #import "ios/web/navigation/navigation_item_impl.h"
 #import "ios/web/navigation/session_storage_builder.h"
+#import "ios/web/navigation/web_view_navigation_proxy.h"
+#import "ios/web/navigation/web_view_navigation_proxy_impl.h"
+#import "ios/web/navigation/wk_based_navigation_manager_impl.h"
 #include "ios/web/public/browser_state.h"
 #import "ios/web/public/crw_session_storage.h"
 #import "ios/web/public/java_script_dialog_presenter.h"
@@ -85,7 +88,9 @@ WebStateImpl::WebStateImpl(const CreateParams& params,
     session_storage_builder.ExtractSessionState(this, session_storage);
   } else {
     navigation_manager_ = base::WrapUnique<NavigationManagerImpl>(
-        new LegacyNavigationManagerImpl);
+        web::GetWebClient()->IsSlimNavigationManagerEnabled()
+            ? (NavigationManagerImpl*)(new WKBasedNavigationManagerImpl)
+            : (NavigationManagerImpl*)(new LegacyNavigationManagerImpl));
     certificate_policy_cache_ =
         base::MakeUnique<SessionCertificatePolicyCacheImpl>();
   }
@@ -737,4 +742,8 @@ WebState* WebStateImpl::GetWebState() {
   return this;
 }
 
+WebViewNavigationProxy* WebStateImpl::GetWebViewNavigationProxy() const {
+  return [web_controller_ webViewNavigationProxy];
+}
+
 }  // namespace web