Add a NOTREACHED for same document load with no history item.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 52c0c387f4980c2760f4f1f98f267f606203db3c..211feb3e0620db61ffcba825b9552794064e7d63 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5538,6 +5538,7 @@ void RenderFrameImpl::NavigateInternal(
           // happen.
           if (current_history_item_.isNull()) {
             history_load_type = blink::WebHistoryDifferentDocumentLoad;
+            NOTREACHED();
           } else {
             // Additionally, if the |current_history_item_|'s document
             // sequence number doesn't match the one sent from the browser, it