[Mac] Add locking as speculative fix for X509Certificate crashes.

net/base/x509_certificate_mac.cc

Index: net/base/x509_certificate_mac.cc
diff --git a/net/base/x509_certificate_mac.cc b/net/base/x509_certificate_mac.cc
index a4c37baab0076212828eee03fac29687114d7ba8..e55648bce83b5a548f41805d683d47c8938e8b6d 100644
--- a/net/base/x509_certificate_mac.cc
+++ b/net/base/x509_certificate_mac.cc
@@ -447,6 +447,9 @@ int X509Certificate::Verify(const std::string& hostname, int flags,
   for (size_t i = 0; i < intermediate_ca_certs_.size(); ++i)
     CFArrayAppendValue(cert_array, intermediate_ca_certs_[i]);
 
+  // From here on, only one thread can be active at a time.

[wtc, 2010/04/27 00:06:43]

It would be nice to explain why you break the Verify method
into two parts at this point.

I guess the code before this point can clearly be executed
by multiple threads.  After this point, it's less clear.
And we know for sure that we do not want multiple threads
to call SecTrustEvaluate (line 503) simultaneously.

+  AutoLock lock(lock_);
+
   SecTrustRef trust_ref = NULL;
   status = SecTrustCreateWithCertificates(cert_array, ssl_policy, &trust_ref);
   if (status)