[LayoutNG] avoid some memcpy's

While stepping throught the code, I've noticed that AddOutOfFlowDescendant
was doing a memcpy.

Change-Id: I97fd39e91656dc31a26ae744780ded3ca4ee9be6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663184
Auto-Submit: Aleks Totic <atotic@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670225}

third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.cc

@@ -202,7 +202,7 @@ NGContainerFragmentBuilder::AddOutOfFlowChildCandidate(
 }
 
 NGContainerFragmentBuilder& NGContainerFragmentBuilder::AddOutOfFlowDescendant(
-    NGOutOfFlowPositionedDescendant descendant) {
+    const NGOutOfFlowPositionedDescendant& descendant) {
   oof_positioned_descendants_.push_back(descendant);
   return *this;
 }

third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.h

@@ -136,7 +136,7 @@ class CORE_EXPORT NGContainerFragmentBuilder : public NGFragmentBuilder {
       base::Optional<TextDirection> container_direction = base::nullopt);
 
   NGContainerFragmentBuilder& AddOutOfFlowDescendant(
-      NGOutOfFlowPositionedDescendant descendant);
+      const NGOutOfFlowPositionedDescendant& descendant);
 
   void GetAndClearOutOfFlowDescendantCandidates(
       Vector<NGOutOfFlowPositionedDescendant>* descendant_candidates,
@@ -217,8 +217,9 @@ class CORE_EXPORT NGContainerFragmentBuilder : public NGFragmentBuilder {
     NGOutOfFlowPositionedDescendant descendant;
     LogicalOffset child_offset;  // Logical offset of child's top left vertex.
 
-    NGOutOfFlowPositionedCandidate(NGOutOfFlowPositionedDescendant descendant,
-                                   LogicalOffset child_offset)
+    NGOutOfFlowPositionedCandidate(
+        const NGOutOfFlowPositionedDescendant& descendant,
+        LogicalOffset child_offset)
         : descendant(descendant), child_offset(child_offset) {}
   };