Bug 1470504 - Skip empty paints when looking for the most recent paints in the APZ test data. r=kats

Retained displaylists can produce empty paints, which wasn't the case before.

Differential Revision: https://phabricator.services.mozilla.com/D7343

--HG--
extra : rebase_source : a7cce68d6cde3f11aa838609586fd939d428650b
extra : histedit_source : d4e490a445daf01db5ba87185cd0461095f003d7

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

@@ -54,6 +54,15 @@ function convertTestData(testData) {
   return result;
 }
 
+function getLastNonemptyBucket(buckets) {
+  for (var i = buckets.length - 1; i >= 0; --i) {
+    if (buckets[i].scrollFrames.length > 0) {
+      return buckets[i];
+    }
+  }
+  return null;
+}
+
 // Given APZ test data for a single paint on the compositor side,
 // reconstruct the APZC tree structure from the 'parentScrollId'
 // entries that were logged. More specifically, the subset of the

gfx/layers/apz/test/mochitest/helper_bug1280013.html

@@ -47,7 +47,7 @@ function* test(testDriver) {
   // get the displayport for the subframe
   var utils = SpecialPowers.getDOMWindowUtils(window);
   var contentPaints = utils.getContentAPZTestData().paints;
-  var lastPaint = convertScrollFrameData(contentPaints[contentPaints.length - 1].scrollFrames);
+  var lastPaint = convertScrollFrameData(getLastNonemptyBucket(contentPaints).scrollFrames);
   var foundIt = 0;
   for (var scrollId in lastPaint) {
     if (('contentDescription' in lastPaint[scrollId]) &&