Chrome Version       : 51.0.2672.0 (Official Build) canary (64-bit)
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
URLs (if applicable) : https://memegen.googleplex.com/create?selected_template=evilest_thing_i_can_imagine

This repro uses a Google-only URL, so apologies in advance.

What steps will reproduce the problem?
1. Launch chrome with the --site-per-process flag (enable it via chrome://flags/#enable-site-per-process)
2. Visit https://memegen.googleplex.com/create?selected_template=evilest_thing_i_can_imagine, which apparently contains a cross site iframe
3. Enter some text in the 'top text' text field.
4. Change the cursor focus to the 'middle text' text field.
5. Try to close the tab.

What is the expected result?
The user should see a 'confirm navigation' prompt and be able to close the tab.

What happens instead of that?
No prompt appears. The tab is unkillable but you can still interact with it, and it is not possible to reload the page or navigate anywhere else. Also, it is impossible to restart Chrome unless you first manually kill the process for this tab.