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

tools/valgrind/memcheck/suppressions_mac.txt

 # There are three kinds of suppressions in this file:
 # 1. Third party stuff we have no control over.
 #
 # 2. Intentional unit test errors, stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing.
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system.
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 492 matching lines @@
 }
 {
    bug_18984
    Memcheck:Free
    fun:_ZdlPv
    fun:_ZN7WebCore15CSSSelectorList19adoptSelectorVectorERN3WTF6VectorIPNS_11CSSSelectorELm0EEE
    ...
    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
    fun:_CFRuntimeCreateInstance
    fun:__CFArrayInit
    fun:CFArrayCreateMutableCopy
    fun:_ZNK3net15X509Certificate6VerifyERKSsiPNS_16CertVerifyResultE
    fun:_ZN3net12CertVerifier7Request8DoVerifyEv
    fun:_ZN14RunnableMethodIN3net12CertVerifier7RequestEMS2_FvvE6Tuple0E3RunEv
    fun:-[TaskOperation main]
@@ skipping 140 matching lines @@
    fun:sqlite3_exec
    fun:sqlite3InitCallback
    fun:sqlite3_exec
    fun:sqlite3VdbeExec
    fun:sqlite3Step
    fun:sqlite3_step
    fun:sqlite3_exec
    fun:_ZN7history11URLDatabase27InitKeywordSearchTermsTableEv
    fun:_ZN7history16ArchivedDatabase4InitERK8FilePath
 }