CHECK-fail if a renderer tries to commit a no-op navigation.

An HTTP status code of 204/205 indicates that no document should
actually be committed. Since this should all be handled by PlzNavigate
on the browser side, CHECK-fail if this somehow reaches the renderer.

Content-Disposition: attachment has the same behavior, so also
CHECK-fail if this somehow reaches the renderer.

Bug: 1117282
Change-Id: I83a04f4a68963da0ffebede327380365e6f9852e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380604Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802492}

third_party/blink/renderer/core/loader/frame_loader.cc

@@ -917,17 +917,19 @@ static bool ShouldNavigate(WebNavigationParams* params, LocalFrame* frame) {
     return true;
 
   int status_code = params->response.HttpStatusCode();
-  if (status_code == 204 || status_code == 205) {
-    // The server does not want us to replace the page contents.
-    return false;
-  }
-
+  // If the server sends 204 or 205, this means the server does not want to
+  // replace the page contents. However, PlzNavigate should have handled it
+  // browser-side and never sent a commit request to the renderer.
+  if (status_code == 204 || status_code == 205)
+    CHECK(false);
+
+  // If the server attached a Content-Disposition indicating that the resource
+  // is an attachment, this is actually a download. However, PlzNavigate should
+  // have handled it browser-side and never sent a commit request to the
+  // renderer.
   if (IsContentDispositionAttachment(
           params->response.HttpHeaderField(http_names::kContentDisposition))) {
-    // The server wants us to download instead of replacing the page contents.
-    // Downloading is handled by the embedder, but we still get the initial
-    // response so that we can ignore it and clean up properly.
-    return false;
+    CHECK(false);
   }
 
   const String& mime_type = params->response.MimeType();
@@ -936,6 +938,8 @@ static bool ShouldNavigate(WebNavigationParams* params, LocalFrame* frame) {
   PluginData* plugin_data = frame->GetPluginData();
   bool can_load_with_plugin = !mime_type.IsEmpty() && plugin_data &&
                               plugin_data->SupportsMimeType(mime_type);
+  // TODO(dcheng): Ideally, this should commit a failed navigation or something,
+  // instead of silently ignoring the commit request.
   if (!can_load_with_plugin) {
     WebLocalFrameImpl::FromFrame(frame)->Client()->WillFailCommitNavigation(
         WebLocalFrameClient::CommitFailureReason::kNoPluginForMimeType);