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/metrics/include/metrics_collections.h

@@ -353,9 +353,13 @@ class EngineMetrics {
   ~EngineMetrics();
 
   // Add a new SessionMetrics instance and return a pointer to the caller.
-  // The new SessionMetrics instance is owned by this EngineMetrics instance
-  // and will exist until RemoveSession is called or this object is deleted.
-  SessionMetrics *AddSession();
+  // A shared_ptr is used since it's possible that the SessionMetrics instance
+  // may be in use after the EngineMetrics instance is closed. The EngineMetrics
+  // instance will hold a shared_ptr reference to the SessionMetrics instance
+  // until RemoveSession is called, the EngineMetrics instance is deleted, or
+  // the SessionMetrics instance is marked as completed and ConsolidateSessions
+  // removes it.
+  std::shared_ptr<SessionMetrics> AddSession();
 
   // Removes the metrics object for the given session id. This should only
   // be called when the SessionMetrics instance is no longer in use.