add Make factory to SkMemoryStream (simplify call-sites)

Bug: skia:6888
Change-Id: Ia4e432673ed089a91229697c8dde0489f220000d
Reviewed-on: https://skia-review.googlesource.com/26884
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
10 files changed