Move SkPDFStream back to SkStream to save memory.

SkPDFStream stores data as a SkStreamRewindable to minimize
deep duplication and memory overhead.

SkStreamToStreamRewindable function to deal with fact that
SkTypeface returns a SkStream.

BUG=skia:2743
R=djsollen@google.com, mtklein@google.com, bungeman@google.com, reed@google.com

Author: halcanary@google.com

Review URL: https://codereview.chromium.org/387863005
4 files changed