Auto merge of #2209 - salty-horse:ignore_delete_document, r=glennw
Auto merge of #2209 - salty-horse:ignore_delete_document, r=glennw

Ignore DeleteDocument during playback of last frame to avoid crashing

This fixes the crash during "replay" when reaching the last frame.
The render backend gets sent a `DeleteDocument`, so it delete its document, and then wrench asks it to `UpdateDocument`.

If we do let it accept the `DeleteDocument`, and have wrench detect it and do no more updates, then we won't be able to rewind. Ignoring it is the simplest option.

Does this break anything? Can there be recordings with multiple documents that are added and deleted as it goes? From what I've seen, wrench adds its own document when it starts.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/webrender/2209)
<!-- Reviewable:end -->

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender