Twenty Seventeen: Customizer preview display issues in IE9

[laurelfulford]

When previewing the static front page of the theme using IE9, the header image displays very large and zoomed in:

This is an issue with how IE9 understands vh when inside of an iframe. Normally IE9 will understand vh as the vertical height relative to the viewport, but in an iframe, IE9 thinks the vertical height is relative to the size of the whole page.

[laurelfulford]

Created some IE9-specific styles in 38722.patch​ that are only loaded in the Customizer. They use pixels to set the height of the custom header and panels, rather than vh.

[davidakennedy]

In 39215:

Twenty Seventeen: Fix Customizer preview display issues in IE9

IE9 doesn't understands vh (vertical height) when inside of an iframe. Normally IE9 will understand vh as the vertical height relative to the viewport, but in an iframe, IE9 thinks the vertical height is relative to the size of the whole page. So this fixes that when the site is viewed in the Customizer preview – an iframe.

Props laurelfulford.

Fixes #38722.