Revert "Avoid WeakMember in observer sets in ImageResourceContent."

This reverts commit 741d5c86b03864874c1a7c62ff7a9549ade6b335.

Reason for revert: suspected of causing failures on linux leak bot
animations/cross-fade-border-image-source.html
animations/cross-fade-webkit-mask-box-image.html

https://ci.chromium.org/ui/p/chromium/builders/ci/WebKit%20Linux%20Leak/62776/overview

Original change's description:
> Avoid WeakMember in observer sets in ImageResourceContent.
>
> Use Member instead.
>
> ImageResourceContent::RemoveObserver() is called when an observer is to
> be removed, which will remove the observer from the set that it's in,
> and perform other operations. Letting oilpan remove entries on its own
> will only cause trouble.
>
> crrev.com/c/5453623 wants PhysicalFragment to take ownership over
> LayoutObject in some cases, and if the PhysicalFragment is in a tree
> that's no longer reachable via trace, neither will the LayoutObject,
> which means that oilpan would be free to remove entries from the
> observer sets. When the LayoutObject later on gets destroyed,
> ImageResourceContent::RemoveObserver() would naturally fail to find the
> observer in any of the two sets.
>
> WeakMember was added in crrev.com/c/3153179 without explaining why that
> was chosen over Member.
>
> Change-Id: Id0e8562a961388bb3ce425a724ba2f5c156ab9f3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5464304
> Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
> Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1289429}

Change-Id: Id4caf0692bb4e684b6220c72dbb3af28611e6261
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463535
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289534}