Auto merge of #16428 - stshine:box-sizing, r=emilio

layout: Floor to 0 when computing intrinsic sizes with box-sizing:border-box

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Improves bitbucket.

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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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tests/wpt/mozilla/meta/MANIFEST.json

@@ -3131,6 +3131,18 @@
      {}
     ]
    ],
+   "css/intrinsic_border_box.html": [
+    [
+     "/_mozilla/css/intrinsic_border_box.html",
+     [
+      [
+       "/_mozilla/css/intrinsic_border_box_ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/issue-1324.html": [
     [
      "/_mozilla/css/issue-1324.html",
@@ -8085,6 +8097,11 @@
      {}
     ]
    ],
+   "css/intrinsic_border_box_ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/issue-1324-ref.html": [
     [
      {}
@@ -22132,6 +22149,14 @@
    "c78c6c992e47adb47ba00b621210ae8564d46389",
    "support"
   ],
+  "css/intrinsic_border_box.html": [
+   "261487f82f350ab020f49f362993b26711a89b5b",
+   "reftest"
+  ],
+  "css/intrinsic_border_box_ref.html": [
+   "9a8f19331c748658b71c85b823b5b3db68e4b7a4",
+   "support"
+  ],
   "css/issue-1324-ref.html": [
    "ef2253d5045622f54634dbfa6b6abdef6b6e00a7",
    "support"