Bug 1692507 [wpt PR 27610] - Fix expectations of a recently committed test, a=testonly

Automatic update from web-platform-tests
Fix (#27610)

In PR #27520 we landed a new test for buttons with children with percentage heights called percentage-descendant-of-anonymous-flex-item.html

In the review process the height: 100% was replaced by height: 100px. However that was a bad decision because every UA styles buttons in a different way (even the same UA in different platforms) meaning that due to margins and paddings that height could be smaller than 100px. Switching back to the percentage height in the expectations.
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wpt-commits: f3a7512a062994d1d2f4f0fbc7fe89c011e6ee4a
wpt-pr: 27610

testing/web-platform/tests/css/css-flexbox/reference/percentage-descendant-of-anonymous-flex-item-ref.html

@@ -4,5 +4,5 @@
 <link rel="author" title="Sergio Villar Senin" href="mailto:svillar@igalia.com">
 <p>The test PASS if you see a 200x100 green rectangle inside a button.</p>
 <button style="width: 200px; height: 100px;">
-    <div style="width: 200px; height: 100px; background-color: green;"></div>
+    <div style="width: 200px; height: 100%; background-color: green;"></div>
 </button>