Bug 1512838 - Skip over empty transaction buckets in isLayerized. r=botond

Sometimes we can get empty transactions after a scrollframe is
layerized. In such cases the isLayerized check would incorrectly detect
the scrollframe as not being layerized because it would only look at the
data for the empty transaction.

Differential Revision: https://phabricator.services.mozilla.com/D23495

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/test/mochitest/apz_test_utils.js

@@ -57,6 +57,9 @@ function convertTestData(testData) {
   return result;
 }
 
+// Returns the last bucket that has at least one scrollframe. This
+// is useful for skipping over buckets that are from empty transactions,
+// because those don't contain any useful data.
 function getLastNonemptyBucket(buckets) {
   for (var i = buckets.length - 1; i >= 0; --i) {
     if (buckets[i].scrollFrames.length > 0) {
@@ -126,8 +129,9 @@ function findRcdNode(apzcTree) {
 // element, and not in the content descriptions of other elements.
 function isLayerized(elementId) {
   var contentTestData = SpecialPowers.getDOMWindowUtils(window).getContentAPZTestData();
-  ok(contentTestData.paints.length > 0, "expected at least one paint");
-  var seqno = contentTestData.paints[contentTestData.paints.length - 1].sequenceNumber;
+  var nonEmptyBucket = getLastNonemptyBucket(contentTestData.paints);
+  ok(nonEmptyBucket != null, "expected at least one nonempty paint");
+  var seqno = nonEmptyBucket.sequenceNumber;
   contentTestData = convertTestData(contentTestData);
   var paint = contentTestData.paints[seqno];
   for (var scrollId in paint) {