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_old_usage_table_ref.h

@@ -12,11 +12,11 @@
 
 #include <stdint.h>
 #include <map>
+#include <mutex>
 #include <string>
 #include <vector>
 
 #include "OEMCryptoCENC.h"
-#include "lock.h"
 #include "oemcrypto_types.h"
 #include "openssl/sha.h"
 
@@ -89,7 +89,7 @@ class OldUsageTable {
 
   typedef std::map<std::vector<uint8_t>, OldUsageTableEntry *> EntryMap;
   EntryMap table_;
-  wvcdm::Lock lock_;
+  std::mutex lock_;
   int64_t generation_;
   CryptoEngine *ce_;