Prevent font-loading promise rejection while shutting down the document

This fixes a crash bug where cancellation of font loading causes promise
rejection inside a ScriptForbiddenScope, when the loading is cancelled
inside Document::Shutdown().

To prevent this, this patch adds an early return to
RemoteFontFaceSource::NotifyFinished() when the document is not active.

This is a short term fix to be merged for M76.

Bug: 960290
Change-Id: I9f8b18fe489c2224ebbf445ad25445b7769c3336
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1661313
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670005}

third_party/blink/renderer/core/css/remote_font_face_source.cc

@@ -117,6 +117,11 @@ void RemoteFontFaceSource::NotifyFinished(Resource* resource) {
   ExecutionContext* execution_context = font_selector_->GetExecutionContext();
   if (!execution_context)
     return;
+  // Prevent promise rejection while shutting down the document.
+  // See crbug.com/960290
+  if (execution_context->IsDocument() &&
+      To<Document>(execution_context)->IsDetached())
+    return;
 
   FontResource* font = ToFontResource(resource);
   histograms_.RecordRemoteFont(font);