Make a single if_contains function to simplify the interface

tests/parallel_hash_map_test.cc

@@ -14,19 +14,12 @@ TEST(THIS_TEST_NAME, ThreadSafeContains) {
     // Test that the nodes have the proper API.
     ThisMap<int, int> m = { {1, 7}, {2, 9} };
     auto val = 0;
-    EXPECT_TRUE(m.if_contains(1, val));
-    EXPECT_EQ(val, 7);
 
-    EXPECT_FALSE(m.if_contains(3, val));
-
-#if __cplusplus > 199711L
     auto func = [&val](int& v) { val = v; };
     EXPECT_TRUE(m.if_contains(2, func));
     EXPECT_EQ(val, 9);
 
     EXPECT_FALSE(m.if_contains(3, func));
-#endif
-
 }
 
 }  // namespace