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/cdm/core/include/crypto_session.h

@@ -7,13 +7,13 @@
 
 #include <map>
 #include <memory>
+#include <mutex>
 #include <string>
 #include <vector>
 
 #include "OEMCryptoCENC.h"
 #include "disallow_copy_and_assign.h"
 #include "key_session.h"
-#include "lock.h"
 #include "metrics_collections.h"
 #include "oemcrypto_adapter.h"
 #include "timer_metric.h"
@@ -278,7 +278,7 @@ class CryptoSession {
 
   static const size_t kAes128BlockSize = 16;  // Block size for AES_CBC_128
   static const size_t kSignatureSize = 32;    // size for HMAC-SHA256 signature
-  static Lock crypto_lock_;
+  static std::mutex crypto_lock_;
   static bool initialized_;
   static int session_count_;