Fix return value of ThreadPool::RecentAverageDelay()
Fix return value of ThreadPool::RecentAverageDelay()

imotai/bfs,linyvxiang/dfs,yvxiang/coverage_for_bfs,yvxiang/coverage_for_bfs,yvxiang/coverage_for_bfs,imotai/bfs,00k/ToyDFS,baidu/bfs,bluebore/dfs,yvxiang/dfs,yvxiang/dfs,myawan/bfs-1,myawan/bfs-1,lylei/dfs,linyvxiang/dfs,yvxiang/dfs,myawan/bfs-1,baidu/bfs,00k/ToyDFS,myawan/bfs-1,bluebore/bfs,lylei/dfs,linyvxiang/dfs,yvxiang/coverage_for_bfs,lylei/dfs,bluebore/dfs,imotai/bfs,bluebore/bfs,bluebore/bfs,bluebore/dfs,baidu/bfs,lylei/dfs,baidu/bfs,yvxiang/dfs