can't scroll in the embedded demo page of iron-collapse (ios 10.1.1)

### Description
can't scroll in the embedded demo page of iron-collapse after expanding the content.

### Expected outcome
You can scroll to see all content.

### Actual outcome
No Scrolling possible.

### Live Demo
https://beta.webcomponents.org/element/PolymerElements/iron-collapse/demo/demo/index.html

### Steps to reproduce
1. Use an iPhone
2. Go to the demo page of the iron-collapse. The content must not use all of the screen (use link above)
3. Toggle the iron-collapse the get all the content. The content must use all of the screen
4. Try to scroll

### Browsers Affected

- [ ] Chrome
- [ ] Firefox
- [x] Safari for iOS
- [ ] Safari 9
- [ ] Safari 8
- [ ] Safari 7
- [ ] Edge
- [ ] IE 11
- [ ] IE 10