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/test/cdm_session_unittest.cpp

@@ -174,7 +174,8 @@ class CdmSessionTest : public WvCdmTestBase {
  protected:
   void SetUp() override {
     WvCdmTestBase::SetUp();
-    cdm_session_.reset(new CdmSession(NULL, &metrics_));
+    metrics_.reset(new metrics::SessionMetrics);
+    cdm_session_.reset(new CdmSession(NULL, metrics_));
     // Inject testing mocks.
     license_parser_ = new MockCdmLicense(cdm_session_->session_id());
     cdm_session_->set_license_parser(license_parser_);
@@ -192,7 +193,7 @@ class CdmSessionTest : public WvCdmTestBase {
     cdm_session_.reset();
   }
 
-  metrics::SessionMetrics metrics_;
+  std::shared_ptr<metrics::SessionMetrics> metrics_;
   std::unique_ptr<CdmSession> cdm_session_;
   MockCdmLicense* license_parser_;
   metrics::CryptoMetrics crypto_metrics_;