Add MD5::Digest methods to create a hex string.

We were doing this in several places throughout the code, so might
as well centralize the logic into one place.

Change-Id: Ie664ca3ce91f1835e9fe47eafd6812062b69fe26
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/718636
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
9 files changed