[iOS] Clear WebUI when navigating back to restore URL

With SlimNavigationManager enabled, navigating back to a restore URL after a
cold launch DCHECKs because the webUI is not cleared.

Bug: 996221
Change-Id: I2c754f2e95dd6fad3cb7861eaff275602154671d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773700Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691815}

ios/web/web_state/ui/crw_web_controller.mm

@@ -834,6 +834,8 @@ typedef void (^ViewportStateCompletion)(const web::PageViewportState*);
     return;
   }
 
+  self.webStateImpl->ClearWebUI();
+
   // This navigation can be an iframe navigation, but it's not possible to
   // distinguish it from the main frame navigation, so context still has to be
   // created.