Auto merge of #29799 - stshine:align-self-stretch, r=Loirooriol

layout_2020: Specify the used cross size when replaced item is stretched

According to spec, if the flex item has [align-self: stretch](https://drafts.csswg.org/css-flexbox/#propdef-align-self), redo layout for its contents, treating this used size as its definite cross size so that percentage-sized children can be resolved.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29788  (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes OR

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-aspect-ratio-img-column-001.html.ini

@@ -0,0 +1,2 @@
+[flex-aspect-ratio-img-column-001.html]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-aspect-ratio-img-row-006.html.ini

@@ -1,2 +0,0 @@
-[flex-aspect-ratio-img-row-006.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-height-flex-items-004.xht.ini

@@ -0,0 +1,2 @@
+[flex-minimum-height-flex-items-004.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-height-flex-items-007.xht.ini

@@ -0,0 +1,2 @@
+[flex-minimum-height-flex-items-007.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-height-flex-items-008.xht.ini

@@ -0,0 +1,2 @@
+[flex-minimum-height-flex-items-008.xht]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-height-flex-items-020.html.ini

@@ -0,0 +1,2 @@
+[flex-minimum-height-flex-items-020.html]
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-width-flex-items-012.html.ini

@@ -1,2 +0,0 @@
-[flex-minimum-width-flex-items-012.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flexbox-definite-cross-size-constrained-percentage.html.ini

@@ -1,2 +0,0 @@
-[flexbox-definite-cross-size-constrained-percentage.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flexbox-whitespace-handling-001a.xhtml.ini

@@ -1,2 +0,0 @@
-[flexbox-whitespace-handling-001a.xhtml]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flexbox-whitespace-handling-001b.xhtml.ini

@@ -1,2 +0,0 @@
-[flexbox-whitespace-handling-001b.xhtml]
-  expected: FAIL