Bug 1967169 - Explicitly set layout.scroll.disable-pixel-alignment to test_bug583889.html. r=dlrobertson

Differential Revision: https://phabricator.services.mozilla.com/D250096

layout/base/tests/test_bug583889.html

@@ -26,6 +26,12 @@ function* runTest() {
   // Wait for onLoad event.
   yield;
 
+  SpecialPowers.pushPrefEnv({
+    set: [ ["layout.scroll.disable-pixel-alignment", true] ]
+  }).then(grabEventAndGo);
+  // Wait for the pref change.
+  yield;
+
   var inner = $("inner");
   inner.src = "bug583889_inner1.html";
   window.onmessage = grabEventAndGo;