Use the std::chrono to get the time.

[ Merge of http://go/wvgerrit/67985 ]

Now that we can use C++11, we should use the cross-platform types for
clocks instead of the platform-specific versions.

Test: WV unit/integration tests.
Change-Id: I50318e3d1caf9e814f33f497f83c19c9f3c154a1

libwvdrmengine/cdm/metrics/include/timer_metric.h

@@ -1,7 +1,7 @@
 #ifndef WVCDM_METRICS_TIMER_METRIC_H_
 #define WVCDM_METRICS_TIMER_METRIC_H_
 
-#include <stdint.h>
+#include <chrono>
 
 namespace wvcdm {
 namespace metrics {
@@ -23,8 +23,8 @@ class TimerMetric {
   double AsUs() const;
 
  private:
-  double sec_;
-  double usec_;
+  std::chrono::steady_clock clock_;
+  std::chrono::time_point<std::chrono::steady_clock> start_;
   bool is_started_;
 
 };