SkDocument: use `auto` for MakeDocument().

A later CL will make this return a unique_ptr<SkDocument>.

Bug: skia:5972
Change-Id: Ie10d6c07d5f2524ecb71d906db0d37427827225d
Reviewed-on: https://skia-review.googlesource.com/c/181660
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
15 files changed