LibWeb: Fix not working Element::scroll_an_element_into_view()

Fixes following mistakes:
- "scrolling box" for a document is not `scrollable_overflow_rect()`
   but size of viewport (initial containing block, like spec says).
- comparing edges of "scrolling box" with edges of target element
  does not make any sense because "scrolling box" edges are relative
  to page while result of `get_bounding_client_rect()` is relative
  to viewport.

Tests/LibWeb/Text/input/scroll-into-view-start.html

@@ -0,0 +1,29 @@
+<script src="include.js"></script>
+<style>
+    body {
+        margin: 0;
+    }
+
+    #box {
+        width: 200px;
+        height: 200px;
+        background-color: magenta;
+    }
+</style>
+<div style="height: 1000px"></div>
+<div style="margin-left: 500px" onclick="clickHandler(event)" id="box"></div>
+<div style="height: 1000px"></div>
+<script>
+    function clickHandler(event) {
+        document.getElementById("box").scrollIntoView({ block: "start" });
+    }
+
+    asyncTest(done => {
+        document.addEventListener("scroll", event => {
+            println("The page has been scrolled to y: " + window.scrollY);
+            done();
+        });
+
+        document.getElementById("box").dispatchEvent(new Event("click"));
+    });
+</script>