REPRO STEPS:

1. a.com embeds a sandboxed b.com subframe:
   <iframe sandbox href=”https://b.com/subframe.html”>

   With site-per-process b.com's frame will be hosted
   in a process dedicated to the b.com’s site.

2. Let’s assume that no unsandboxed frames are present in
   the b.com's process.

EXPECTED BEHAVIOR: Even if b.com’s process gets compromised, it should not be able to:
     - Create new windows or dialogs
     - Load plugins
     - Submit forms
     - Navigate the parent frame
     - Lock the mouse pointer
     (this list is based on https://www.html5rocks.com/en/tutorials/security/sandboxed-iframes/)

ACTUAL BEHAVIOR: No browser-side enforcement of iframe-sandbox attributes?