Revert of Use registerWeakMembers to clean up IntersectionObserver. (patchset #1 id:1 of https://codereview.chromium.org/1591763003/ )

Reason for revert:
Broke on some tests w/ Oilpan enabled,

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

backing it out.

(see also https://codereview.chromium.org/1596333002/ )

Original issue's description:
> 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
>
> Committed: https://crrev.com/546573354e47ddd01f491c382701fa51500f85eb
> Cr-Commit-Position: refs/heads/master@{#369948}

TBR=dcheng@chromium.org,haraken@chromium.org,szager@chromium.org
BUG=540528
NOTRY=true

Review URL: https://codereview.chromium.org/1600243002

Cr-Commit-Position: refs/heads/master@{#369970}
2 files changed