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.cpp

@@ -166,7 +166,7 @@ OldUsageTable::OldUsageTable(CryptoEngine *ce) {
 }
 
 OldUsageTableEntry *OldUsageTable::FindEntry(const std::vector<uint8_t> &pst) {
-  wvcdm::AutoLock lock(lock_);
+  std::unique_lock<std::mutex> lock(lock_);
   return FindEntryLocked(pst);
 }
 
@@ -192,13 +192,13 @@ OldUsageTableEntry *OldUsageTable::CreateEntry(
     return NULL;
   }
   OldUsageTableEntry *entry = new OldUsageTableEntry(this, pst_hash);
-  wvcdm::AutoLock lock(lock_);
+  std::unique_lock<std::mutex> lock(lock_);
   table_[pst_hash] = entry;
   return entry;
 }
 
 void OldUsageTable::Clear() {
-  wvcdm::AutoLock lock(lock_);
+  std::unique_lock<std::mutex> lock(lock_);
   for (EntryMap::iterator i = table_.begin(); i != table_.end(); ++i) {
     if (i->second) delete i->second;
   }