Backport 1b55a64 to 2.1 - Fixes #6818

[AntonEvers]

Please add this to the 2.1.8 release if it can be made in time.

Fixed Issues (if relevant)

1. PageCache gives error "Uncaught TypeError: element.prop is not a function" when there is an iframe #6818: PageCache gives error "Uncaught TypeError: element.prop is not a function" when there is an iframe

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[hostep]

Not sure if this is enough to fix it, maybe these changes are also necessary:
#8005

I'm totally not sure about this, there are a lot of comments spreading across multiple issues and PR's discussing very similar problems. I just want to make sure the correct code is being backported :)

[AntonEvers]

@hostep I have added them, will test on my own machine now.

[AntonEvers]

@hostep this works just as well indeed.