Auto merge of #25645 - mrobinson:scrollable-overflow-v2, r=SimonSapin

Add support for overflow:scroll and overflow:hidden to layout_2020

This adds clipping and interactive scrolling support, but scrolling from
script is still not functional.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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tests/wpt/metadata-layout-2020/css/css-backgrounds/background-repeat/background-repeat-repeat-x.xht.ini

@@ -1,2 +0,0 @@
-[background-repeat-repeat-x.xht]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-backgrounds/background-repeat/background-repeat-repeat-y.xht.ini

@@ -1,2 +0,0 @@
-[background-repeat-repeat-y.xht]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-backgrounds/background-size/background-size-contain.xht.ini

@@ -1,2 +0,0 @@
-[background-size-contain.xht]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-backgrounds/background-size/background-size-cover.xht.ini

@@ -1,2 +0,0 @@
-[background-size-cover.xht]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/acid2_ref.html.ini

@@ -1,2 +0,0 @@
-[acid2_ref.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/border_radius_clip_a.html.ini

@@ -1,2 +0,0 @@
-[border_radius_clip_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/css-position-3/position-sticky-bottom.html.ini

@@ -1,2 +0,0 @@
-[position-sticky-bottom.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/css-position-3/position-sticky-top.html.ini

@@ -1,2 +0,0 @@
-[position-sticky-top.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/overflow_auto.html.ini

@@ -1,2 +0,0 @@
-[overflow_auto.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/overflow_position_abs_simple_a.html.ini

@@ -1,2 +0,0 @@
-[overflow_position_abs_simple_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/overflow_scroll.html.ini

@@ -1,2 +0,0 @@
-[overflow_scroll.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/overflow_simple_a.html.ini

@@ -1,2 +0,0 @@
-[overflow_simple_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/overflow_xy_a.html.ini

@@ -1,2 +0,0 @@
-[overflow_xy_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/text_overflow_ellipsis.html.ini

@@ -1,2 +0,0 @@
-[text_overflow_ellipsis.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/text_overflow_string.html.ini

@@ -1,2 +0,0 @@
-[text_overflow_string.html]
-  expected: FAIL