Restore: Sync Gutenberg @ 8c78d87453509661a9f28f978ba2c242d515563b

See changelog in 21d3c57a1b46afb2f19be23db3acd55a392c664d

> Editor: Bump pinned hash for the Gutenberg repository.
> This updates the pinned hash from the `gutenberg` from `9b8144036fa5faf75de43d4502ff9809fcf689ad` to `8c78d87453509661a9f28f978ba2c242d515563b`.
>
> The following changes are included:

git-svn-id: https://develop.svn.wordpress.org/branches/fixes-64393-restore-version-history@62135 602fd350-edb4-49c9-b593-d223f7449a82