Describe the bug
Clicking "Convert to Blocks" successfully converts classic editor content to blocks, and appears to add to the undo history, but clicking "undo" has no result.

In my testing this is caused by oEmbed lines in post_content pointing to WP posts.

To Reproduce
Steps to reproduce the behavior:

1. Create a post in the Classic Editor with a WordPress oEmbed link, e.g.…

My paragraph

https://www.chrisvanpatten.com/notes-from-wordcamp-halifax-2018

My other paragraph

2. Open the post in Gutenberg
3. Open the Classic Editor block's settings menu and select "Convert to Blocks"
4. Note that the Undo button is enabled
5. Try to click undo
6. See nothing happen

Expected behavior
Either this process should be "undoable" or the undo button shouldn't change its state.

Screenshots

Desktop (please complete the following information):

• OS: macOS High Sierra 10.13.5
• Browser: Safari 11.1.1
• Gutenberg Version: 3.0.1