remove unused dynamicwstream.snapshotAsData()

Checking to invalidate this on every write() call has a measurable cost, so removing it both simplifies the class and speeds it up.

BUG=skia:

Change-Id: Idf0baa265c9a0b5d26d82fce948c61ed9b0810b1
Reviewed-on: https://skia-review.googlesource.com/6096
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed