app-header-layout should respect header's scrollTarget

[bennypowers]

Description

<app-header-layout> clobbers it's header's scrollTarget in _updateLayoutStates

Expected outcome

If scrollTarget is set on header, it should be respected

Actual outcome

scrollTarget is reset to either documentElement or contentContainer

Live Demo

https://jsbin.com/wusagubagi/1/edit?html,js,console

Steps to reproduce

1. Put a app-header-layout element in the page with an app-header.

• set the app-header scrollTarget property.
• scrollTarget is reset by app-header-layout.

Browsers Affected

• Chrome
• [ x] Firefox
• [ x] Safari 9
• [ x] Safari 8
• [x ] Safari 7
• [x ] Edge
• [x ] IE 11
• [x ] IE 10