Oilpan: Fix weak processing for IntersectionObserver::m_root

This is a follow-up fix for https://codereview.chromium.org/1591763003/.
IntersectionObserver::m_root must be cleared in the weak callback when the m_root is dead.

BUG=

Review URL: https://codereview.chromium.org/1594813002

Cr-Commit-Position: refs/heads/master@{#369956}
2 files changed