script: Prevent "scroll to fragment" from scrolling offscreen (#32129)

Previously, the "scroll to fragment" operation could scroll past the end
of the screen, because the scroll position was not clamped to viewport
boundaries. Correct this by using the `Window::scroll()` method which
handles this case.

In addition, ensure that `Window`'s `current_viewport` member is
initialized properly when it is created.

tests/wpt/meta/MANIFEST.json

@@ -621660,6 +621660,13 @@
          }
         ]
        ],
+       "scroll-position-inline-nearest.html": [
+        "4aab0aa5e5a1e4a3eea82282be2344586a9a6d02",
+        [
+         null,
+         {}
+        ]
+       ],
        "scroll-position-vertical-lr.html": [
         "57d99440e114968e7dcd1b61ebf2d18c7bca987b",
         [

tests/wpt/meta/html/browsers/browsing-the-web/scroll-to-fragid/scroll-position-vertical-lr.html.ini

@@ -0,0 +1,3 @@
+[scroll-position-vertical-lr.html]
+  [Fragment Navigation: Scroll to block start position in vertical-lr writing mode]
+    expected: FAIL