remove SkWriteBuffer::getWriter32().

remove SkWriteBuffer::getWriter32().

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=1936563002

TBR=reed@google.com
Just deleting API

Review-Url: https://codereview.chromium.org/1936563002