Move the scrollbar with `translateY` in modern browsers

This will improve scrollbar animation performance with content-heavy elements. Facebook's ticker suffers from this.
Steal the feature detection from modernizr for it (if it has one)

No file or test is named. Start by locating the scrollbar animation implementation, then investigate whether Modernizr provides the required feature detection for translateY. Done means modern browsers use translateY for scrollbar animation while existing browser behavior remains supported.

Written by the indexing model from the issue text.