Switch to using shared_ptr for Session Metrics

[ Merge from http://go/wvgerrit/71443 ]

The assumption that the metrics will always outlive the CdmSession
instance appears not to always hold (at least in a non-android
multi-threaded solution). The shared_ptr ensures that the metrics
are available even in these rare race conditions.

BUG: http://b/123321465
Test: CDM unit tests. Also http://go/wvgerrit/71264 parallel tests.
Change-Id: Iaa6a8f6c0fdc46a911789759d6e1228d849aa237

libwvdrmengine/cdm/core/src/cdm_engine.cpp

@@ -133,8 +133,8 @@ CdmResponseType CdmEngine::OpenSession(
 
   CloseExpiredReleaseSessions();
 
-  std::unique_ptr<CdmSession> new_session(new CdmSession(file_system_,
-                                                    metrics_->AddSession()));
+  std::unique_ptr<CdmSession> new_session(
+      new CdmSession(file_system_, metrics_->AddSession()));
   CdmResponseType sts = new_session->Init(property_set, forced_session_id,
                                           event_listener);
   if (sts != NO_ERROR) {