Bug 1797011 Part 3 - Delete nsFrameList's copy assigment operator. r=emilio

This patch doesn't change behavior.

Differential Revision: https://phabricator.services.mozilla.com/D160015

layout/base/nsCSSFrameConstructor.cpp

@@ -3843,7 +3843,7 @@ void nsCSSFrameConstructor::ConstructFrameFromItemInternal(
           childList.DestroyFrames();
         }
 
-        childList = newList;
+        childList = std::move(newList);
       }
 
       if (!(bits & FCDATA_ALLOW_GRID_FLEX_COLUMN) ||

layout/generic/nsBlockFrame.cpp

@@ -5626,7 +5626,7 @@ void nsBlockFrame::SetOverflowOutOfFlows(nsFrameList&& aList,
   } else if (HasAnyStateBits(NS_BLOCK_HAS_OVERFLOW_OUT_OF_FLOWS)) {
     NS_ASSERTION(aPropValue == GetProperty(OverflowOutOfFlowsProperty()),
                  "prop value mismatch");
-    *aPropValue = aList;
+    *aPropValue = std::move(aList);
   } else {
     SetProperty(OverflowOutOfFlowsProperty(),
                 new (PresShell()) nsFrameList(std::move(aList)));

layout/generic/nsFrameList.h

@@ -101,10 +101,7 @@ class nsFrameList {
   // nsFrameList is a move-only class by default. Use Clone() if you really want
   // a copy of this list.
   nsFrameList(const nsFrameList& aOther) = delete;
-
-  // XXX: ideally, copy assignment should be removed because we should use move
-  // assignment to transfer the ownership.
-  nsFrameList& operator=(const nsFrameList& aOther) = default;
+  nsFrameList& operator=(const nsFrameList& aOther) = delete;
   nsFrameList Clone() const { return nsFrameList(mFirstChild, mLastChild); }
 
   /**