SkWriteBuffer exposes its lower-level implementation SkWriter32 through
this one call. It's not currently used in any interesting way:
- write_encoded_bitmap() uses it to manually re-create writeDataAsByteArray();
- unit tests use it incidentally as a quick way to read the serialized bytes.
This should be SkWriteBuffer no longer necessarily needs to have an SkWriter32.
Landing this will let us then remove SkWriter32::contiguousArray().
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1936563002TBR=reed@google.com
Just deleting API
Review-Url: https://codereview.chromium.org/1936563002