[web] Rename x/yCoord to AnchorPoint in TooltipPositionStyle

Summary: Follow up: https://phab.comm.dev/D4915#inline-32530

Test Plan: Run web app, confirm that the rename doesn't break tooltip behavior

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5039