Don't allow link navigations for invalid URLs.

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index a1f5aa0211eef998ec0e47ebedb88e7325a85d75..5b5067b64d770efc0c576afc1b9a00de2227d07f 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -4963,6 +4963,18 @@ - (void)webView:(WKWebView*)webView
   // 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) {