Update the suppression for issue 20459 with Chris Hawk's findings....

tools/valgrind/memcheck/suppressions_mac.txt

Index: tools/valgrind/memcheck/suppressions_mac.txt
===================================================================
--- tools/valgrind/memcheck/suppressions_mac.txt	(revision 25183)
+++ tools/valgrind/memcheck/suppressions_mac.txt	(working copy)
@@ -510,7 +510,11 @@
    fun:_Z10cssyyparsePv
 }
 {
-   # Leak of a CFMutableArray of certs in X509Certificate::Verify.
+   # Leak of a CFMutableArray of certs in Apple's
+   # Security::KeychainCore::Trust::evaluate(), which is called by
+   # SecTrustEvaluate().  The leak occurs when we precede a call to
+   # SecTrustEvaluate() with a call to SecTrustSetAnchorCertificates(), which
+   # we do only in the unit tests (to trust the test root CA certificate).
    bug_20459
    Memcheck:Leak
    fun:malloc_zone_malloc