code: @GuardedBy("mLock")
    private boolean maybeUpdateFileCacheSize() {
      long now = mClock.now();
      if ((!mCacheStats.isInitialized()) ||
          mCacheSizeLastUpdateTime == UNINITIALIZED ||
          (now - mCacheSizeLastUpdateTime) > FILECACHE_SIZE_UPDATE_PERIOD_MS) {
        return maybeUpdateFileCacheSizeAndIndex();
      }
      return false;
    }
docs: If file cache size is not calculated or if it was calculated
  a long time ago (FILECACHE_SIZE_UPDATE_PERIOD_MS) recalculated from file listing.
  @return true if it was recalculated, false otherwise.