Bug 1427635: Fix DoCompareTreePosition frame tree version with null aCommonAncestor. r=xidorn

It doesn't fill the ancestors of the first frame if aCommonAncestor is
null, which means that we get garbage afterwards.

MANUAL PUSH: Relanding of old patch.

layout/base/nsLayoutUtils.cpp

@@ -1291,8 +1291,11 @@ int32_t nsLayoutUtils::DoCompareTreePosition(
   }
 
   AutoTArray<nsIFrame*, 20> frame1Ancestors;
-  if (aCommonAncestor &&
-      !FillAncestors(aFrame1, aCommonAncestor, &frame1Ancestors)) {
+  // Note that the order of the condition is important. We need to fill the
+  // ancestors even if aCommonAncestor is null, otherwise the code below makes
+  // no sense.
+  if (!FillAncestors(aFrame1, aCommonAncestor, &frame1Ancestors) &&
+      aCommonAncestor) {
     // We reached the root of the frame tree ... if aCommonAncestor was set,
     // it is wrong
     return DoCompareTreePosition(aFrame1, aFrame2, aIf1Ancestor, aIf2Ancestor,