Make Digest (renamed Md5Digest) a little more multi-purpose.

--
MOS_MIGRATED_REVID=130986194
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/TreeArtifactValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/TreeArtifactValue.java
index e4ee468..b4fe3ee 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/TreeArtifactValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/TreeArtifactValue.java
@@ -22,7 +22,7 @@
 import com.google.common.collect.Maps;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.actions.Artifact.TreeFileArtifact;
-import com.google.devtools.build.lib.actions.cache.Digest;
+import com.google.devtools.build.lib.actions.cache.DigestUtils;
 import com.google.devtools.build.lib.actions.cache.Metadata;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -69,7 +69,7 @@
     }
 
     return new TreeArtifactValue(
-        Digest.fromMetadata(digestBuilder).asMetadata().digest,
+        DigestUtils.fromMetadata(digestBuilder).getDigestBytesUnsafe(),
         ImmutableMap.copyOf(childFileValues));
   }