Added mutex for reserved_license_ids_.

[ Merge of http://go/wvgerrit/150630 ]

DeviceFiles uses a static variable for tracking license IDs which
have been reserved by a CDM session before officially storing the
license on the device.  This variable was not protected by a mutex,
and a rare race condition would arise, either crashing the service
or getting it stuck in a loop.

This CL adds a mutex for protecting the set of reserved IDs.

Bug: 226555704
Test: device_files_unittest
Change-Id: Icdea88673c76c267b4b7db79697ec52ae8e2581e

libwvdrmengine/cdm/core/include/device_files.h

@@ -5,6 +5,7 @@
 #ifndef WVCDM_CORE_DEVICE_FILES_H_
 #define WVCDM_CORE_DEVICE_FILES_H_
 
+#include <mutex>
 #include <set>
 #include <string>
 #include <vector>
@@ -369,6 +370,7 @@ class DeviceFiles {
 #endif
 
   static std::set<std::string> reserved_license_ids_;
+  static std::mutex reserved_license_ids_mutex_;
 
   wvutil::FileSystem* file_system_;
   CdmSecurityLevel security_level_;