scrollTop and scrollLeft always 0 in DOMContentLoaded handler on a scrolled page

[Ahmad-S792]

scrollTop and scrollLeft always 0 in DOMContentLoaded handler on a scrolled page
https://bugs.webkit.org/show_bug.cgi?id=250814

Reviewed by NOBODY (OOPS!).

Merge - https://chromium.googlesource.com/chromium/blink/+/3ad81cf0b724a2a25ee5cc79a09541d9a971ae15

Currently, when performing a "back" navigation, the scroll position
is restored at the time the first layout happens (see
FrameLoader::didFirstLayout). If that does not go through, another
attempt to restore the scroll position is made when the frame
completes loading (see FrameLoader::checkLoadCompleteForThisFrame).

In a closer look, one might notice that in the later, not only the
"back" navigation case is covered, but also "reload".

Patch makes these two attempts of restore the scroll position to
operate under the same circumstances: back and reload.

Apart from bringing them in pair, it also makes it possible to
query the scroll position at the time DOMContentLoaded fires, as
Gecko and Blink (Presto in the past as well) engines do.

* Source/WebCore/loader/FrameLoader.cpp:
(FrameLoader::didFirstLayout):
(1) Add early return, if not presnet "m_frame.page()"
(2) Accounted for Reload types during back navigation
* LayoutTests/fast/loader/scroll-position-restored-on-reload-at-load-event.html: Add Test Case
* LayoutTests/fast/loader/scroll-position-restored-on-reload-at-load-event-expected.txt: Add Test Case Expectation
* LayoutTests/fast/loader/fast/loader/scroll-position-restored-on-back-at-load-event.html: Add Test Case
* LayoutTests/fast/loader/fast/loader/scroll-position-restored-on-back-at-load-event-expected.txt: Add Test Case Expectation

a592af3

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[webkit-early-warning-system]

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[webkit-early-warning-system]

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✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🛠 gtk	✅ 🛠 wincairo
✅ 🧪 webkitperl	🧪 ios-wk2	✅ 🧪 api-mac	❌ 🧪 gtk-wk2
	✅ 🧪 api-ios	❌ 🧪 mac-wk1	🧪 api-gtk
	✅ 🛠 tv	❌ 🧪 mac-wk2
	✅ 🛠 tv-sim	❌ 🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
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[webkit-early-warning-system]

EWS run on previous version of this PR (hash bd2c879)

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✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	❌ 🛠 gtk
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	❌ 🧪 gtk-wk2
	✅ 🧪 api-ios	❌ 🧪 mac-wk1	❌ 🧪 api-gtk
	✅ 🛠 tv	❌ 🧪 mac-wk2
	✅ 🛠 tv-sim	❌ 🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 8619852)

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

[webkit-early-warning-system]

EWS run on current version of this PR (hash a592af3)

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