[GridNG] Update test file to match reference scrollbars

The reference for this test is 100x100-grey-box-with-scrollbars.html,
which has a 150px by 150px square overflowing a 100px by 100px box, but
the test specifies a 120px by 120px square overflowing a 100px by 100px
box. Since the scrollbar thumb is based on the amount of content inside
of the scrollable region, the size of the scrollbar thumb didn't match
the reference and the test was failing.

The fix here is to make the test 150px by 150px so it matches the
reference. The conditions that it's testing (that scrollbars are
visible) remains unchanged.

Bug: 1335889
Change-Id: If78b7d9a2ad04bde5b6e1ce1f7757e537ab97535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5077836
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1231543}

css/css-grid/grid-model/grid-areas-overflowing-grid-container-009.html

@@ -10,7 +10,7 @@
 <link href="/css/support/grid.css" rel="stylesheet">
 <style>
     .grid {
-       grid: 120px / 120px;
+       grid: 150px / 150px;
        width: 100px;
        height: 100px;
        overflow: auto;