Version: 53.0.2778.0
OS: Windows, Linux

What steps will reproduce the problem?
1) Start debug build of Chrome with --site-per-process and open Task Manager.
2) Visit http://csreis.github.io/tests/cross-site-iframe.html.
3) Click "Go cross-site (simple page)"
4) Right click inside the iframe and open DevTools.  (Important, so that we're
inspecting the OOPIF's page.)
5) Type "while (1);" in the console.
6) Try to select text in the iframe and wait 20 seconds.

What is the expected output?

The unresponsiveness dialog should be shown.

What do you see instead?

The browser crashes on a DCHECK in WebContentsEntry::OnRendererUnresponsive:
DCHECK_EQ(render_widget_host->GetProcess(),
          web_contents()->GetMainFrame()->GetProcess());

This isn't a safe assumption to make-- any widget in the tab might become unresponsive, even those in out-of-process iframes.  Looks like the bug is that we end up notifying other code (the processes extension API) that the main frame's process is unresponsive when it's really a different frame's process.

(Note that fixing this bug probably won't be sufficient to get the unresponsiveness dialog to appear for OOPIFs; we can file another bug if needed.)