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/metrics/include/event_metric.h

@@ -7,13 +7,13 @@
 
 #include <cstdarg>
 #include <map>
+#include <mutex>
 #include <string>
 #include <vector>
 
 #include "OEMCryptoCENC.h"
 #include "attribute_handler.h"
 #include "distribution.h"
-#include "lock.h"
 #include "log.h"
 #include "metrics.pb.h"
 #include "pow2bucket.h"
@@ -50,7 +50,7 @@ class BaseEventMetric {
    * This locks the internal state of the event metric to ensure safety across
    * multiple threads preventing the caller from worrying about locking.
    */
-  Lock internal_lock_;
+  std::mutex internal_lock_;
 };
 
 // The EventMetric class is used to capture statistics about an event such as