Add a UseCounter for position: sticky

This patch adds a UseCounter for position: sticky by tracking when new
StickyConstraints are added. This feat is achieved by removing non-const
access to PaintLayerScrollableArea::GetStickyConstraintsMap and adding
an explicit "AddStickyConstraints" function that increments the counter.

Bug: 1109232
Change-Id: Ic0543c6e3501019ada58aeec2c3885c3854c7897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317607


Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791493}