perf(virtual-scroll): improve scrolling performance

[mmalerba]

• Adds some CSS properties to help improve performance
• Changes spacer element to use a transform rather than width/height to expand the viewport

[crisbeto]

LGTM

[crisbeto]

I might be a good idea to try this out on IE. I remember it doing some weird stuff when it comes to sizing and transform.

[mmalerba]

Seems to work fine

[jelbourn]

LGTM

[ngbot]

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