Updated metric Distribution and Timer utils.

[ Merge of http://go/wvgerrit/137811 ]

Renamed TimerMetric to Timer.  Timer is used to generate durations
included in metrics, but is not a metric itself.  The method of
getting the current time did not require creating an instance of
std::steady_clock.

Updated Distribution and Timer to use default initializers instead of
constructor initialization list.

Bug: 204946540
Test: Metric unit tests
Change-Id: I7ed291b586347dd0b7ab305960883bec04637315

libwvdrmengine/cdm/metrics/include/timer_metric.h

@@ -7,10 +7,9 @@
 
 namespace wvcdm {
 namespace metrics {
-class TimerMetric {
+class Timer {
  public:
-  // Constructs a new TimerMetric.
-  explicit TimerMetric() : is_started_(false) {}
+  Timer() {}
   // Starts the clock running. If the clock was previously set, this resets it.
   // IsStarted will return true after this call.
   void Start();
@@ -25,9 +24,8 @@ class TimerMetric {
   double AsUs() const;
 
  private:
-  std::chrono::steady_clock clock_;
   std::chrono::time_point<std::chrono::steady_clock> start_;
-  bool is_started_;
+  bool is_started_ = false;
 };
 }  // namespace metrics
 }  // namespace wvcdm