Re-use the precomputed universe key in WalkableGraphFactory#prepareAndGet.

--
MOS_MIGRATED_REVID=139357569
diff --git a/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java b/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
index 240ea7c..50de277 100644
--- a/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
+++ b/src/main/java/com/google/devtools/build/skyframe/WalkableGraph.java
@@ -92,8 +92,8 @@
 
   /** Provides a WalkableGraph on demand after preparing it. */
   interface WalkableGraphFactory {
-    EvaluationResult<SkyValue> prepareAndGet(Collection<String> roots, String offset,
-        int numThreads, EventHandler eventHandler) throws InterruptedException;
+    EvaluationResult<SkyValue> prepareAndGet(
+        SkyKey universeKey, int numThreads, EventHandler eventHandler) throws InterruptedException;
 
     /** Returns the {@link SkyKey} that defines this universe. */
     SkyKey getUniverseKey(Collection<String> roots, String offset);