Do not load about:blank if iframe requested to load invalid URL.

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 4756 matching lines @@
     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.
+  // the invalid navigation and load about:blank if navigation was requested for
+  // the main frame.
   GURL requestURL = net::GURLWithNSURL(action.request.URL);
   if (!requestURL.is_valid()) {
     decisionHandler(WKNavigationActionPolicyCancel);
-    GURL aboutBlankURL(url::kAboutBlankURL);
-    web::NavigationManager::WebLoadParams loadParams(aboutBlankURL);
-    loadParams.referrer = [self currentReferrer];
-    self.webState->GetNavigationManager()->LoadURLWithParams(loadParams);
+    if (action.targetFrame.mainFrame) {
+      GURL aboutBlankURL(url::kAboutBlankURL);
+      web::NavigationManager::WebLoadParams loadParams(aboutBlankURL);
+      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];
     }
@@ skipping 762 matching lines @@
   }
 
   return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
 }
 
 - (NSString*)refererFromNavigationAction:(WKNavigationAction*)action {
   return [action.request valueForHTTPHeaderField:@"Referer"];
 }
 
 @end