Revert of Oilpan: Fix weak processing for IntersectionObserver::m_root (patchset #3 id:40001 of https://codereview.chromium.org/1594813002/ )

Reason for revert:
This and the parent change introduced some failures,

 https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Oilpan%20Leak/builds/16410

backing out for now.

Original issue's description:
> 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=
>
> Committed: https://crrev.com/2760f06a3c43c8919f4ecae43f460ada20d0990b
> Cr-Commit-Position: refs/heads/master@{#369956}

TBR=szager@chromium.org,haraken@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/1596333002

Cr-Commit-Position: refs/heads/master@{#369969}
2 files changed