[web-test] Fix test that assumes DOMRectList has writable elements

The web-test "non-fast-scrollable-region-nested.html" layout test
passes a DOMRectList directly to Array#sort. This CL spreads the
DOMRectList into an actual JSArray, making it sortable.

R=adamk@chromium.org

Bug: v8:8714
Change-Id: Iaa380a7522a3e65f922e42720f278c0236305c53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627386
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663019}

third_party/blink/web_tests/scrollingcoordinator/non-fast-scrollable-region-nested.html

@@ -24,7 +24,7 @@
               'iframe are correctly offset by the iframe location.');
 
   onload = function() {
-    nonFastScrollableRects = sortRects(internals.nonFastScrollableRects(document));
+    nonFastScrollableRects = sortRects([...internals.nonFastScrollableRects(document)]);
     shouldBe('nonFastScrollableRects.length', '3');
     shouldBeEqualToString('rectToString(nonFastScrollableRects[0])', '[51, 102, 200, 200]');
     shouldBeEqualToString('rectToString(nonFastScrollableRects[1])', '[51, 402, 211, 211]');