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/ActionMetadataHandler.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
index dd93e04..087e97a 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionMetadataHandler.java
@@ -23,7 +23,7 @@
 import com.google.devtools.build.lib.actions.ActionInputHelper;
 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.Md5Digest;
 import com.google.devtools.build.lib.actions.cache.Metadata;
 import com.google.devtools.build.lib.actions.cache.MetadataHandler;
 import com.google.devtools.build.lib.skyframe.TreeArtifactValue.TreeArtifactException;
@@ -264,11 +264,11 @@
   }
 
   @Override
-  public void setDigestForVirtualArtifact(Artifact artifact, Digest digest) {
+  public void setDigestForVirtualArtifact(Artifact artifact, Md5Digest md5Digest) {
     Preconditions.checkArgument(artifact.isMiddlemanArtifact(), artifact);
-    Preconditions.checkNotNull(digest, artifact);
-    additionalOutputData.put(artifact,
-        FileArtifactValue.createProxy(digest.asMetadata().digest));
+    Preconditions.checkNotNull(md5Digest, artifact);
+    additionalOutputData.put(
+        artifact, FileArtifactValue.createProxy(md5Digest.getDigestBytesUnsafe()));
   }
 
   private Set<TreeFileArtifact> getTreeArtifactContents(Artifact artifact) {