Don't allow link navigations for invalid URLs.

ios/web/web_state/ui/crw_web_controller.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/web_state/ui/crw_web_controller.h"
 
 #import <WebKit/WebKit.h>
 
 #import <objc/runtime.h>
 #include <stddef.h>
@@ skipping 4945 matching lines @@
   _webProcessIsDead = NO;
   if (_isBeingDestroyed) {
     decisionHandler(WKNavigationActionPolicyCancel);
     return;
   }
 
   // The page will not be changed until this navigation is commited, so the
   // retrieved state will be pending until |didCommitNavigation| callback.
   [self updatePendingNavigationInfoFromNavigationAction:action];
 
+  // Invalid URLs should not be loaded.  However, simply doing nothing upon
+  // tapping a link or button is a jarring user experience.  Instead, cancel
+  // the invalid navigation and load about:blank.
+  GURL requestURL = net::GURLWithNSURL(action.request.URL);
+  if (!requestURL.is_valid()) {

[Eugene But (OOO till 7-30), 2016/10/05 00:58:39]

What |requestURL.is_valid()| would return for "about:blank" URLs? If the result
is false, then we can get into infinite loading loop.

[kkhorimoto, 2016/10/05 01:16:26]

GURL("about:blank").is_valid() is true, so that's not an issue here.

+    decisionHandler(WKNavigationActionPolicyCancel);
+    web::NavigationManager::WebLoadParams loadParams(GURL("about:blank"));

[Eugene But (OOO till 7-30), 2016/10/05 01:41:44]

s/"about:blank"/kAboutBlankURL

[kkhorimoto, 2016/10/05 04:09:19]

Done.

+    loadParams.referrer = [self currentReferrer];
+    self.webState->GetNavigationManager()->LoadURLWithParams(loadParams);
+    return;
+  }
+
   BOOL allowLoad = [self shouldAllowLoadWithNavigationAction:action];
 
   if (allowLoad) {
     allowLoad = self.webStateImpl->ShouldAllowRequest(action.request);
     if (!allowLoad && action.targetFrame.mainFrame) {
       [_pendingNavigationInfo setCancelled:YES];
     }
   }
 
   decisionHandler(allowLoad ? WKNavigationActionPolicyAllow
@@ skipping 656 matching lines @@
   }
 
   return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
 }
 
 - (NSString*)refererFromNavigationAction:(WKNavigationAction*)action {
   return [action.request valueForHTTPHeaderField:@"Referer"];
 }
 
 @end