[UIProcess] Unsafe downcast in ScrollingTreeStickyNode::computeLayerPosition leading to type confusion.

[JonWBedard]

f2967e8

[UIProcess] Unsafe downcast in ScrollingTreeStickyNode::computeLayerPosition leading to type confusion.
https://bugs.webkit.org/show_bug.cgi?id=271395
rdar://125084284

Reviewed by Chris Dumez.

Fix various places in scrolling tree-related code where we do unchecked `downcast<>`s of
scrolling tree types.

* Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp:
(WebCore::AsyncScrollingCoordinator::frameViewVisualViewportChanged):
(WebCore::AsyncScrollingCoordinator::setViewportConstraintedNodeConstraints):
* Source/WebCore/page/scrolling/ScrollingStateOverflowScrollProxyNode.cpp:
(WebCore::ScrollingStateOverflowScrollProxyNode::dumpProperties const):
* Source/WebCore/page/scrolling/ScrollingStateStickyNode.cpp:
(WebCore::ScrollingStateStickyNode::computeLayerPosition const):
* Source/WebCore/page/scrolling/ScrollingTreeOverflowScrollProxyNode.cpp:
(WebCore::ScrollingTreeOverflowScrollProxyNode::dumpProperties const):
* Source/WebCore/page/scrolling/ScrollingTreeStickyNode.cpp:
(WebCore::ScrollingTreeStickyNode::computeLayerPosition const):

Originally-landed-as: 272448.773@safari-7618-branch (052845bc6c5c). rdar://128088754
Canonical link: https://commits.webkit.org/278914@main

9e21554

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	⏳ 🛠 wpe-skia
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash cc0243e)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	❌ 🛠 ios	❌ 🛠 mac	🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	❌ 🛠 ios-sim	❌ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	❌ 🧪 ios-wk2	❌ 🧪 api-mac	🧪 api-wpe
	❌ 🧪 ios-wk2-wpt	❌ 🧪 mac-wk1	⏳ 🛠 wpe-skia
	❌ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🛠 gtk
	❌ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	❌ 🧪 gtk-wk2
	🛠 tv-sim	❌ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
	❌ 🛠 watch
	🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash af16092)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	❌ 🛠 ios	❌ 🛠 mac	❌ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	❌ 🛠 ios-sim	❌ 🛠 mac-AS-debug	❌ 🧪 wpe-wk2
✅ 🧪 webkitperl	❌ 🧪 ios-wk2	❌ 🧪 api-mac	❌ 🧪 api-wpe
	❌ 🧪 ios-wk2-wpt	❌ 🧪 mac-wk1	⏳ 🛠 wpe-skia
	❌ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🛠 gtk
	❌ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	❌ 🧪 gtk-wk2
	🛠 tv-sim	❌ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
	❌ 🛠 watch
	🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash f762cb4)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	❌ 🛠 ios	❌ 🛠 mac	❌ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	❌ 🛠 ios-sim	❌ 🛠 mac-AS-debug	❌ 🧪 wpe-wk2
✅ 🧪 webkitperl	❌ 🧪 ios-wk2	❌ 🧪 api-mac	❌ 🧪 api-wpe
	❌ 🧪 ios-wk2-wpt	❌ 🧪 mac-wk1	⏳ 🛠 wpe-skia
	❌ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🛠 gtk
	❌ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	❌ 🧪 gtk-wk2
	🛠 tv-sim	❌ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
	❌ 🛠 watch
	🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 9e21554)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	⏳ 🛠 wpe-skia
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 gtk-wk2
	✅ 🛠 tv-sim	✅ 🧪 mac-wk2-stress	❌ 🧪 api-gtk
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 278914@main (f2967e8): https://commits.webkit.org/278914@main

Reviewed commits have been landed. Closing PR #28663 and removing active labels.