Avoid a data copy inside user_image_loader.cc

Use more move semantics inside user_image_loader.cc so DecodeImage()
owns the data. Then DecodeImage() can pass it to ImageDecoder using move
semantics.

Along the way:
- Use span / base::MakeRefCounted() in UserImageRequest ctor.
- Document UserImageRequest is self-deleting and make its dtor private.
- Remove a TODO for moving the data into UserImageRequest, as that is
  impossible to achieve.

Bug: 41243179
Change-Id: Icc654049b25d4886201378ea4a8a25df87fbd4d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5835907
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1351575}