Two-line fix to allow proper embedding via iframes

I'm running into an issue where embedding this fork of neuroglancer via iframes captures the focus, and makes the renders the rest of the page unusable. Would it be possible to cherry-pick the following commit which should solve this issue? Thanks!

https://github.com/google/neuroglancer/commit/18d29e9bf2108fceaf3d9358069b388a5c21b995