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/include/cdm_session.h

@@ -35,7 +35,8 @@ class CdmSession {
   // and |metrics| parameters. Both parameters are owned by the caller and
   // must remain in scope througout the scope of the new instance. |metrics|
   // must not be null.
-  CdmSession(FileSystem* file_system, metrics::SessionMetrics* metrics);
+  CdmSession(FileSystem* file_system,
+             std::shared_ptr<metrics::SessionMetrics> metrics);
   virtual ~CdmSession();
 
   void Close() { closed_ = true; }
@@ -198,7 +199,7 @@ class CdmSession {
 
   virtual CdmResponseType GetDecryptHashError(std::string* hash_error_string);
 
-  virtual metrics::SessionMetrics* GetMetrics() { return metrics_; }
+  virtual metrics::SessionMetrics* GetMetrics() { return metrics_.get(); }
 
  private:
   friend class CdmSessionTest;
@@ -223,7 +224,7 @@ class CdmSession {
   void set_file_handle(DeviceFiles* file_handle);
 
   // instance variables
-  metrics::SessionMetrics* metrics_;
+  std::shared_ptr<metrics::SessionMetrics> metrics_;
   metrics::CryptoMetrics* crypto_metrics_;
   metrics::TimerMetric life_span_;
   metrics::TimerMetric license_request_latency_;