Reland my ClientSocketPool refactor again...

base/stl_util-inl.h

Index: base/stl_util-inl.h
diff --git a/base/stl_util-inl.h b/base/stl_util-inl.h
index 54b7b296c0ad2adc76bb9261535b95bf6d4fd9f2..70fa69e57d7bb415799ade34de914a7214f945a4 100644
--- a/base/stl_util-inl.h
+++ b/base/stl_util-inl.h
@@ -447,4 +447,11 @@ std::vector<T> SetToVector(const std::set<T>& values) {
   return result;
 }
 
+// Test to see if a set, map, hash_set or hash_map contains a particular key.
+// Returns true if the key is in the collection.
+template <typename Collection, typename Key>
+bool ContainsKey(const Collection& collection, const Key& key) {
+  return collection.find(key) != collection.end();
+}
+
 #endif  // BASE_STL_UTIL_INL_H_