Don't load WebUI if provisional navigation is for non-chrome URL

Putative fix for crbug.com/545479. If WebUIManager is created but
navigation occurs before the WebUI URL starts loading, provisional
navigation for a non-WebUI URL could be observed by WebUIManager, and
WebUI should not be loaded in that case.

BUG=545479

Review URL: https://codereview.chromium.org/1464383002

Cr-Commit-Position: refs/heads/master@{#361143}

ios/web/webui/crw_web_ui_manager.mm

@@ -88,6 +88,10 @@ const char kScriptCommandPrefix[] = "webui";
 - (void)webState:(web::WebState*)webState
     didStartProvisionalNavigationForURL:(const GURL&)URL {
   DCHECK(webState == _webState);
+  // If URL is not an application specific URL, ignore the navigation.
+  if (!web::GetWebClient()->IsAppSpecificURL(URL))
+    return;
+
   GURL navigationURL(URL);
   // Add request group ID to the URL, if not present. Request group ID may
   // already be added if restoring state to a WebUI page.