Navigation: Making NavigationClient work with fallback content.

A corner case was not supported yet with the NavigationClient, only
running interface callbacks with FrameNavigationControl, which caused
a connection DCHECK.

Bug: 784904
Change-Id: I9dbec944b47584c737c14d4e604d999ca19d50c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503735
Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638522}

content/renderer/render_frame_impl.cc

@@ -3633,8 +3633,11 @@ void RenderFrameImpl::CommitFailedNavigationInternal(
       }
       Send(new FrameHostMsg_DidStopLoading(routing_id_));
     } else {
-      if (callback)
+      if (callback) {
         std::move(callback).Run(blink::mojom::CommitResult::Ok);
+      } else {
+        navigation_client_impl_.reset();
+      }
     }
     browser_side_navigation_pending_ = false;
     browser_side_navigation_pending_url_ = GURL();