Remove unused ContiguousContainer methods and cleanups

- Remove unused ContiguousContainer methods such as Clear(), Swap(),
  etc.
- Replace RemoveLast() with ReplaceLastByMoving(). This removes the
  logic to deal with removed items, making ContiguousContainer
  appending only.
- Let ContiguousContainer store Buffer directly in the vector instead of
  through unique_ptr.
- Use 'item' instead of 3 different terms 'item', 'element', 'object' in
  ContiguousContainer
- Remove DisplayItem::IsMovedFromCachedSubsequence() because with the
  new PaintChunkSubset we can use
  PaintChunk.is_moved_from_cached_subsequence.
- etc.

Bug: 917911
Change-Id: I8b3004d001ea12a00fed96fc3e031986db1ee005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501064


Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822912}