Moved window opening callback to WebStateDelegate.

ios/web/web_state/web_state_delegate.mm

Index: ios/web/web_state/web_state_delegate.mm
diff --git a/ios/web/web_state/web_state_delegate.mm b/ios/web/web_state/web_state_delegate.mm
index 67a12bc8a6aea7cb28b973b2c9be3ba22f9c6012..b3f3faad8bbee2524f5ecb9d5dd1769136b9e43a 100644
--- a/ios/web/web_state/web_state_delegate.mm
+++ b/ios/web/web_state/web_state_delegate.mm
@@ -18,6 +18,13 @@ WebStateDelegate::~WebStateDelegate() {
   DCHECK(attached_states_.empty());
 }
 
+WebState* WebStateDelegate::CreateNewWebState(WebState* source,
+                                              const GURL& url,
+                                              const GURL& opener_url,
+                                              bool initiated_by_user) {
+  return nullptr;
+}
+
 WebState* WebStateDelegate::OpenURLFromWebState(
     WebState*,
     const WebState::OpenURLParams&) {