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

net/base/x509_certificate.h

Index: net/base/x509_certificate.h
diff --git a/net/base/x509_certificate.h b/net/base/x509_certificate.h
index 32c16f16bcc521c28e6b7b3b6f9ffade2eee13e5..f53035ada531df9e475b968e88e03bd6a06ca5cb 100644
--- a/net/base/x509_certificate.h
+++ b/net/base/x509_certificate.h
@@ -293,6 +293,11 @@ class X509Certificate : public base::RefCountedThreadSafe<X509Certificate> {
   OSCertHandles intermediate_ca_certs_;
 #endif
 
+#if defined(OS_MACOSX)
+  // Blocks multiple threads from verifying the cert simultaneously.
+  mutable Lock lock_;

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

Nit: perhaps rename this lock "verify_lock_" to stress the
fact that it applies to the Verify() method only.

Nit: should we explain why this lock is mutable? (because
Verify is a const method)

+#endif
+
   // Where the certificate comes from.
   Source source_;