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_session.cpp

@@ -27,7 +27,7 @@ const size_t kKeySetIdLength = 14;
 namespace wvcdm {
 
 CdmSession::CdmSession(FileSystem* file_system,
-                       metrics::SessionMetrics* metrics)
+                       std::shared_ptr<metrics::SessionMetrics> metrics)
     : metrics_(metrics),
       initialized_(false),
       closed_(true),
@@ -47,7 +47,7 @@ CdmSession::CdmSession(FileSystem* file_system,
       usage_entry_number_(0),
       mock_license_parser_in_use_(false),
       mock_policy_engine_in_use_(false) {
-  assert(metrics_);  // metrics_ must not be null.
+  assert(metrics_.get());  // metrics_ must not be null.
   crypto_metrics_ = metrics_->GetCryptoMetrics();
   crypto_session_.reset(CryptoSession::MakeCryptoSession(crypto_metrics_));
   life_span_.Start();
@@ -67,8 +67,8 @@ CdmSession::~CdmSession() {
   }
   Properties::RemoveSessionPropertySet(session_id_);
 
-  if (metrics_) {
-    M_RECORD(metrics_, cdm_session_life_span_, life_span_.AsMs());
+  if (metrics_.get()) {
+    M_RECORD(metrics_.get(), cdm_session_life_span_, life_span_.AsMs());
     metrics_->SetCompleted();
   }
 }