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/src/event_metric.cpp

@@ -10,7 +10,7 @@ namespace wvcdm {
 namespace metrics {
 
 BaseEventMetric::~BaseEventMetric() {
-  AutoLock lock(internal_lock_);
+  std::unique_lock<std::mutex> lock(internal_lock_);
 
   for (std::map<std::string, Distribution *>::iterator it = value_map_.begin();
        it != value_map_.end(); it++) {
@@ -19,7 +19,7 @@ BaseEventMetric::~BaseEventMetric() {
 }
 
 void BaseEventMetric::Record(const std::string &key, double value) {
-  AutoLock lock(internal_lock_);
+  std::unique_lock<std::mutex> lock(internal_lock_);
 
   Distribution *distribution;