Repro:

1. In a chrome app, add an iframe pointing to a local web accessible resource containing a webview.
2. Note that it doesn't load.

It's also interesting that:
3. Using devtools, edit the HTML and add a <webview> to the main frame.
4. Not try step 1 again. Notice that the <webview> now loads.

Tested on both browserplugin and oopif-based webview.