Add HasCommitted() check to MHVE::DidFinishNavigation

The RFH accessed in a DCHECK might be null if the navigation never reached the ReadyToCommitNavigation state. This CL updates the check added in https://chromium-review.googlesource.com/c/chromium/src/+/2569637 to be clearer.

Bug: 1133115
Change-Id: I9e4fcc6f239a6e69e855f7abd56b74a3b2cad0cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574118Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834101}

extensions/browser/guest_view/mime_handler_view/mime_handler_view_embedder.cc

@@ -106,12 +106,13 @@ void MimeHandlerViewEmbedder::ReadyToCommitNavigation(
 
 void MimeHandlerViewEmbedder::DidFinishNavigation(
     content::NavigationHandle* handle) {
-  if (!render_frame_host_ ||
+  if (!handle->HasCommitted() ||
       frame_tree_node_id_ != handle->GetFrameTreeNodeId()) {
     return;
   }
   // We should've deleted the MimeHandlerViewEmbedder at this point if the frame
   // is sandboxed.
+  DCHECK(render_frame_host_);
   DCHECK(!render_frame_host_->IsSandboxed(
       network::mojom::WebSandboxFlags::kPlugins));
 }