Thread-safety annotations for the various skyframe graph interfaces.

--
MOS_MIGRATED_REVID=90103351
diff --git a/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java b/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
index e050830..83a225d 100644
--- a/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
+++ b/src/main/java/com/google/devtools/build/skyframe/QueryableGraph.java
@@ -13,6 +13,8 @@
 // limitations under the License.
 package com.google.devtools.build.skyframe;
 
+import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
+
 import java.util.Map;
 import java.util.Set;
 
@@ -21,6 +23,7 @@
 /**
  * A graph that exposes its entries and structure, for use by classes that must traverse it.
  */
+@ThreadSafe
 public interface QueryableGraph {
   /**
    * Returns the node with the given name, or {@code null} if the node does not exist.