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.

libwvdrmengine/cdm/metrics/include/timer_metric.h

@@ -7,13 +7,14 @@ namespace wvcdm {
 namespace metrics {
 
 class TimerMetric {
-
  public:
+  // Constructs a new TimerMetric.
+  explicit TimerMetric() : is_started_(false) {}
   // Starts the clock running. If the clock was previously set, this resets it.
   // IsStarted will return true after this call.
   void Start();
   // Returns whether or not the timer has started.
-  bool IsStarted() const { return is_started_; };
+  bool IsStarted() const { return is_started_; }
   // Stops the clock and clears the current value.  IsStarted will return false
   // after this call.
   void Clear();
@@ -26,7 +27,6 @@ class TimerMetric {
   std::chrono::steady_clock clock_;
   std::chrono::time_point<std::chrono::steady_clock> start_;
   bool is_started_;
-
 };
 
 }  // namespace metrics