[IntersectionObserver] Fix tracking bookkeeping

R=chrishtr@chromium.org

Change-Id: Ibcab6aef237ce0d801efae344c784ce4e9d8f468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1513077
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639717}
4 files changed