Implement the overflow-anchor CSS property as an opt-out for ScrollAnchoring

This CL implements the opt-out described in the ScrollAnchoring explainer:
https://github.com/WICG/interventions/blob/master/scroll-anchoring/explainer.md

Intent to implement:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/TbuxbOtyZvk

BUG=599180

Review-Url: https://codereview.chromium.org/2145823002
Cr-Commit-Position: refs/heads/master@{#405368}
18 files changed