Log whenever a periodic flush of the action cache is slow.

--
MOS_MIGRATED_REVID=119813568
diff --git a/src/main/java/com/google/devtools/build/lib/util/PersistentMap.java b/src/main/java/com/google/devtools/build/lib/util/PersistentMap.java
index 05f805e..2835c5c 100644
--- a/src/main/java/com/google/devtools/build/lib/util/PersistentMap.java
+++ b/src/main/java/com/google/devtools/build/lib/util/PersistentMap.java
@@ -136,7 +136,7 @@
    * Marks the map as dirty and potentially writes updated entries to the
    * journal.
    */
-  private void markAsDirty() {
+  protected void markAsDirty() {
     dirty = true;
     if (updateJournal()) {
       writeJournal();