Use registerWeakMembers to clean up IntersectionObserver.

Pre-oilpan, when the root disappears, it observers are cleaned up
in NodeIntersectionObserverData::dispose().

Post-oilpan, the cleanup will be done with registerWeakMembers.

BUG=540528
R=haraken@chromium.org,dcheng@chromium.org

Review URL: https://codereview.chromium.org/1591763003

Cr-Commit-Position: refs/heads/master@{#369948}
2 files changed