We currently require support for IntersectionObserver.
which is supported on Edge >= 15 (15 has partial support),
Firefox >55, Chrome >58, Safari 12.1, Opera >=38,
iOS Safari >=12.2, Android 100
Full ES6 is supported in Edge >=15, Firefox >=54, Chrome >=51,
Safari >=10, Opera >=38, iOS Safari >=10, so such a change
would only drop support for Edge 15 and Firefox 54.
CSS.escape is guaranteed in all these browsers according to
caniuse, with the only discrepancy being the Edge browser (versions
16-18) so it is also suggested we remove support for those browsers.
Firefox 54 accounts for 0.0026% of page views
Edge 15-18 accounts for 0.069% of page views
Bug: T306355
Change-Id: Id2987e3456607b610c38da9ee157a026d1d00ada
