It would be nice to be able to do serialization directly on a buffer to avoid extra copies.

One case is gfx.mojom.SharedBufferSkBitmap that transports a serialized skia.mojom.Bitmap via shared buffer [1]. With the current vector based api, the code makes 4 copies:

Sender:
- skia::mojom::Bitmap::Serialize to a vector;
- Copy vector to a shared buffer;

Receiver:
- Copy data from the shared buffer to a vector;
- skia::mojom::Bitmap::Deserialize from the vector;

If vector can be taken off from the picture, we save two copies.

[1]: https://cs.chromium.org/chromium/src/ui/gfx/image/mojo/image_skia_struct_traits.cc?rcl=3e92f75741339da0c094fd863bb4a79575a74792&l=26,35,77,81