Bug 1659567 - Add a memory reporter for gAccessCounts. r=froydnj

This table is only in debug builds, but it takes up about 100kb,
so it sticks out in DMD reports taken in debug builds.

Differential Revision: https://phabricator.services.mozilla.com/D87347

modules/libpref/Preferences.cpp

@@ -3136,6 +3136,12 @@ PreferenceServiceReporter::CollectReports(
     sizes.mMisc += mallocSizeOf(gSharedMap);
   }
 
+#ifdef ACCESS_COUNTS
+  if (gAccessCounts) {
+    sizes.mMisc += gAccessCounts->ShallowSizeOfIncludingThis(mallocSizeOf);
+  }
+#endif
+
   MOZ_COLLECT_REPORT("explicit/preferences/hash-table", KIND_HEAP, UNITS_BYTES,
                      sizes.mHashTable, "Memory used by libpref's hash table.");