[LayoutNG] Avoid NGPaintFragment::List::size()

The use here is for memory alloc performance, but since it
has inline capacity of 64, it won't help when |size()| is
O(n).

Bug: 945671
Change-Id: I166e6615bcae5a814116ee1b6aaf0349405a67ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535507
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644431}

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

@@ -788,8 +788,10 @@ const NGInlineBreakToken* NGBlockLayoutAlgorithm::TryReuseFragmentsFromCache(
     const NGPhysicalFragment& fragment;
     NGLogicalOffset offset;
   };
+  // Avoid calling |ReserveInitialCapacity()| because
+  // |lineboxes->Children().size()| is O(n), not linear, but it's critical to
+  // have enough capacity.
   Vector<FragmentWithLogicalOffset, 64> fragments;
-  fragments.ReserveInitialCapacity(lineboxes->Children().size());
   for (const NGPaintFragment* child : lineboxes->Children()) {
     if (child->IsDirty())
       break;