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/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