SkDynamicMemoryWStream::detachAsData more memory efficent

I did some testing with very large datasets, and the difference in max
RSS is measurable and significant.

Change-Id: I6bb2f795d5b4f6ebdba42c3089dc85a278355d48
Reviewed-on: https://skia-review.googlesource.com/9686
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed