Implement scrollbar-gutter on viewports

[lukewarlow]

ccc39c2

Implement scrollbar-gutter on viewports
https://bugs.webkit.org/show_bug.cgi?id=257606

Reviewed by NOBODY (OOPS!).

Updates scrollbar gutter implementation to work for viewports.

* Source/WebCore/platform/ScrollView.cpp:
(WebCore::ScrollView::documentScrollPositionRelativeToViewOrigin const):
(WebCore::ScrollView::verticalScrollbarWidth const):
(WebCore::ScrollView::horizontalScrollbarHeight const):
(WebCore::ScrollView::locationOfContents const):
* Source/WebCore/platform/ScrollView.h:
* Source/WebCore/rendering/RenderBox.cpp:
(WebCore::RenderBox::includeVerticalScrollbarSize const):

ccc39c2

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	✅ 🛠 gtk
	✅ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🧪 gtk-wk2
	✅ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 3a5787d)

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	❌ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	❌ 🧪 mac-wk1	❌ 🧪 gtk-wk2
	✅ 🧪 api-ios	❌ 🧪 mac-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv	❌ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[lukewarlow]

TODO:

• Extract out improved style resolution into separate PR

• Fix build failure

• Work out why the tests fail, (offsetWidth doesn't seem to correctly account for scrollbar gutters)

• Work out why there's a double gutter if a scrollbar is rendering (overflow: scroll or an overflowing viewport)

• Cleanup code some of these changes are probably not neccesary

• Add more tests for viewport scrollbar-gutter usage (such as testing both-edges)

[lukewarlow]

Closing for now will reopen soon. Extracting the improvement of the style resolution into its own PR.

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 2d92071)

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	🛠 gtk
	🧪 ios-wk2-wpt	🧪 mac-wk1	🧪 gtk-wk2
	🧪 api-ios	🧪 mac-wk2	🧪 api-gtk
	❌ 🛠 tv	❌ 🧪 mac-AS-debug-wk2
	🛠 tv-sim	🧪 mac-wk2-stress
	🛠 watch
	❌ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 9c6807b)

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	🛠 gtk
	✅ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🧪 gtk-wk2
	✅ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash ccc39c2)

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	✅ 🛠 gtk
	✅ 🧪 api-ios	❌ 🧪 mac-wk2	❌ 🧪 gtk-wk2
	✅ 🛠 tv	❌ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim

[lukewarlow]

TODO re-evaluate current state of this and work out next steps to completion.