Change from custom Lock to std::mutex.

[ Merge of http://go/wvgerrit/67884 ]

Now that we can use C++11, we should use the cross-platform std::mutex
type, not the custom pthread version.

Bug: 111850982
Test: WV unit/integration tests
Change-Id: If2fde2836826c5184609e6b1f3a6511206bd4594

libwvdrmengine/oemcrypto/ref/src/oemcrypto_engine_ref.h

@@ -11,13 +11,13 @@
 #include <time.h>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <vector>
 
 #include <openssl/rsa.h>
 
 #include "OEMCryptoCENC.h"
 #include "file_store.h"
-#include "lock.h"
 #include "oemcrypto_auth_ref.h"
 #include "oemcrypto_key_ref.h"
 #include "oemcrypto_rsa_key_shared.h"
@@ -201,7 +201,7 @@ class CryptoEngine {
   uint8_t* destination_;
   ActiveSessions sessions_;
   AuthenticationRoot root_of_trust_;
-  wvcdm::Lock session_table_lock_;
+  std::mutex session_table_lock_;
   std::unique_ptr<wvcdm::FileSystem> file_system_;
   std::unique_ptr<UsageTable> usage_table_;