SkDynamicMemoryWStream: optimize detachAsStream() for tiny streams

Empty streams: one heap alloc instead of two.

Single-block streams: realloc to used portion of block.

Change-Id: I3d7b3a465b28409a4bb93782ea1ddb21a97f70a1
Reviewed-on: https://skia-review.googlesource.com/c/161600
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed