This pull request to adds better support for OOP iframes (see #2548)
The current problem with OOP iframes is that they are moved to a different target. Because of this, the previous versions of Puppeteer pretty much ignored them.
This change extends the FrameManager to already take OOP iframes into account and hides the fact that those frames are actually in different targets.
Further work needs to be done to also make the NetworkManager aware of these and to make sure that settings like emulations etc. are also properly passed down to the new targets.
