Use scrollBy instead of scrollTo in a test in scrollTo-scrollBy-snaps…

….html.

Given the test description is mentioning scrollBy, scrollBy should be used
there.

Depends on D21623

Differential Revision: https://phabricator.services.mozilla.com/D21624

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1373835
gecko-commit: 9d201f2c32796be22a0defcca7c47cde48a5fd36
gecko-integration-branch: central
gecko-reviewers: botond

css/css-scroll-snap/scrollTo-scrollBy-snaps.html

@@ -151,9 +151,9 @@
     window.scrollTo(0, 0);
     assert_equals(window.scrollX, 0);
     assert_equals(window.scrollY, 0);
-    window.scrollTo(input);
+    window.scrollBy(input);
     assert_equals(window.scrollX, expectedX);
     assert_equals(window.scrollY, expectedY);
   }, `scrollBy(${format_dict(input)}) on window lands on (${expectedX}, ${expectedY})`);
 });
-</script>
+</script>