Interactions with the BackForwardCache?

[ArthurSonzogni]

We should ensure when the opener enters the BFCache, the pip window won't stay.

Chrome currently do not allow the BFCache to be used if there are more than 1 related windows. So you should get it for free when developing it for Chrome I believe. Still, the correct behavior should be defined.

Options are:

1. PIP document makes the opener ineligible to enter the BFCache. (Simple)
2. Force close the PIP document when entering the BFCache (Somewhat simple, but probably not worth it)
3. Make the pip window to enter with its opener into the bfcache. (Very complex)