Auto merge of #16807 - mrobinson:scroll-clamping, r=emilio

Fix clamping of scroll position in window.scrollBy

For rightward and downward overflow the spec says:

Let x be max(0, min(x, viewport scrolling area width - viewport width)).
Let y be max(0, min(y, viewport scrolling area height - viewport height)).

Previously, those operations were reversed, which created negative
overflow even when the overflow direction was downward. This change
ensures that Servo matches spec behavior.

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- [ ] These changes fix #__ (github issue number if applicable).

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -19783,6 +19783,12 @@
      {}
     ]
    ],
+   "mozilla/scrollBy.html": [
+    [
+     "/_mozilla/mozilla/scrollBy.html",
+     {}
+    ]
+   ],
    "mozilla/scrollTo.html": [
     [
      "/_mozilla/mozilla/scrollTo.html",
@@ -31354,6 +31360,10 @@
    "b247e3a0ba733c1a8b129ce2994b862d8ed3a423",
    "testharness"
   ],
+  "mozilla/scrollBy.html": [
+   "0243d584bc615a73ea1667fc35f5d73b5165d19f",
+   "testharness"
+  ],
   "mozilla/scrollTo.html": [
    "b9917be5fed364dbc46264f641f54f275b5c054a",
    "testharness"