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/core/include/properties.h

@@ -11,7 +11,6 @@
 
 #include "cdm_client_property_set.h"
 #include "disallow_copy_and_assign.h"
-#include "lock.h"
 #include "wv_cdm_types.h"
 
 #if defined(UNIT_TEST)