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/counter_metric.h

@@ -8,12 +8,12 @@
 #include <cstdarg>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <string>
 #include <vector>
 
 #include "attribute_handler.h"
 #include "field_tuples.h"
-#include "lock.h"
 
 namespace wvcdm {
 namespace metrics {
@@ -51,7 +51,7 @@ class BaseCounterMetric {
    * across multiple threads preventing the caller from worrying about
    * locking.
    */
-  Lock internal_lock_;
+  std::mutex internal_lock_;
 };
 
 // The CounterMetric class is used to track a counter such as the number of