Fix demo pages visibility.

[SchnWalter]

Added has-scrolling-region attribute to <app-header-layout>.

See comment: https://github.com/PolymerElements/iron-component-page/pull/122/files#r111762268

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[SchnWalter]

I signed it!

[googlebot]

CLAs look good, thanks!

[e111077]

LGTM