Let VE know if we are running on iOS

Scrolling to section in the editor has to be implemented differently
on iOS (scrolling within a div vs. scrolling the whole page).

Also, force hardware acceleration on .overlay-content to avoid rendering
bugs on iOS Safari.

Related changes (not hard dependencies):
* I6e47edfa8c3a3a46fd7a4f11e4a54955f3694b9b
* I518312b3a038f2e5c9cc2022c51fc4e1e63f599d

Change-Id: Ic1ba7c921f9adb9f535ee4c01aaca47e3e263994