Add @pure annotations to LateDataUtils

Author: kennknowles

runners/core-java/src/main/java/org/apache/beam/runners/core/LateDataUtils.java

@@ -25,6 +25,7 @@
 import org.apache.beam.sdk.values.WindowingStrategy;
 import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.FluentIterable;
 import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
+import org.checkerframework.dataflow.qual.Pure;
 import org.joda.time.Duration;
 import org.joda.time.Instant;
 
@@ -41,6 +42,7 @@ private LateDataUtils() {}
    * Return when {@code window} should be garbage collected. If the window's expiration time is on
    * or after the end of the global window, it will be truncated to the end of the global window.
    */
+  @Pure
   public static Instant garbageCollectionTime(
       BoundedWindow window, WindowingStrategy windowingStrategy) {
     return garbageCollectionTime(window, windowingStrategy.getAllowedLateness());
@@ -50,6 +52,7 @@ public static Instant garbageCollectionTime(
    * Return when {@code window} should be garbage collected. If the window's expiration time is on
    * or after the end of the global window, it will be truncated to the end of the global window.
    */
+  @Pure
   public static Instant garbageCollectionTime(BoundedWindow window, Duration allowedLateness) {
 
     // If the end of the window + allowed lateness is beyond the "end of time" aka the end of the