[Nav Experiment] Improve comments in |loadCurrentURL|.

I missed this suggestion in http://crrev.com/c/1145725

Bug: 865985
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ie656457f7acf0acb5da81cfc267a7962acb62cc3
Reviewed-on: https://chromium-review.googlesource.com/1146902Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577567}

ios/web/web_state/ui/crw_web_controller.mm

@@ -1915,8 +1915,9 @@ registerLoadRequestForURL:(const GURL&)requestURL
   if (!_containerView)
     return;
 
-  // WKBasedNavigationManagerImpl requires web usage to be enabled to load any
-  // URL. So bail if web usage is disabled, and let the URL be loaded when web
+  // WKBasedNavigationManagerImpl needs WKWebView to load native views, but
+  // WKWebView cannot be created while web usage is disabled to avoid breaking
+  // clearing browser data. Bail now and let the URL be loaded when web
   // usage is enabled again. This can happen when purging web pages when an
   // interstitial is presented over a native view. See https://crbug.com/865985
   // for details.