Disable fixed header when keyboard is open in new overlays (iOS)

This is needed because position: fixed doesn't work when keyboard is
open in mobile Safari and the header just stays in the place it was when
the keyboard was opened (covering the content under it). It's the same
fix that was present in the old EditorOverlay.

Change-Id: Ife8950c7706a0ef04035f36e611d1b5a7e9d275d
Merge "Disable fixed header when keyboard is open in new overlays (iOS)"