Bug 1464761 Part 3 - Remove IsTruncated() in nsColumnSetFrame::ReflowChildren(). r=dholbert

In nsReflowStatus::UpdateTruncated(), mTruncated can be true only when
mIsTopOfPage is false. However, when we reflow -moz-column-content, we always
set mIsTopOfPage to true. Therefore, IsTruncated() always returns false, i.e.
`!IsTruncated()` is always true.

Differential Revision: https://phabricator.services.mozilla.com/D151460

layout/generic/nsColumnSetFrame.cpp

@@ -737,7 +737,7 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
     // Build a continuation column if necessary
     nsIFrame* kidNextInFlow = child->GetNextInFlow();
 
-    if (aStatus.IsFullyComplete() && !aStatus.IsTruncated()) {
+    if (aStatus.IsFullyComplete()) {
       NS_ASSERTION(!kidNextInFlow, "next in flow should have been deleted");
       child = nullptr;
       break;
@@ -924,8 +924,7 @@ nsColumnSetFrame::ColumnBalanceData nsColumnSetFrame::ReflowChildren(
     aDesiredSize.UnionOverflowAreasWithDesiredBounds();
   }
 
-  colData.mFeasible =
-      allFit && aStatus.IsFullyComplete() && !aStatus.IsTruncated();
+  colData.mFeasible = allFit && aStatus.IsFullyComplete();
 
   COLUMN_SET_LOG(
       "%s: Done column reflow pass: %s, mMaxBSize=%d, mSumBSize=%d, "