add tests/bench for dynamicwstream

BUG=skia:

Change-Id: I5a995b63b1a2975cce8101717777eaa6fc12af21
Reviewed-on: https://skia-review.googlesource.com/6035
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed