Tell the user what to do with Restricted objects

[Oriol Brufau]

1. Load data:text/html,<iframe sandbox srcdoc="Inspect me"></iframe>
2. Right-click "Inspect me" and choose "Inspect Element"
3. Switch to the console, enter $0.

A Restricted object is shown. Reps should show some kind of message explaining that the targeted document is not allowed to fully access that object, and that only a subset (usually none) of the properties will be shown. It should suggest using "Select an iframe as the current targeted document" to select the appropriate document or using console.log($0) to log the object using its global as the debuggee.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1404027

[Nicolas Chevobbe]

This issue was moved to firefox-devtools/debugger#6120