Bug 1921288 - Add a way to get the associated duration type from TimeStamp and AwakeTimeStamp. r=gsvelto

Differential Revision: https://phabricator.services.mozilla.com/D223792

mozglue/misc/AwakeTimeStamp.h

@@ -30,6 +30,7 @@ class AwakeTimeDuration;
 // methods are equivalent on non-Windows.
 class AwakeTimeStamp {
  public:
+  using DurationType = AwakeTimeDuration;
   MFBT_API static AwakeTimeStamp NowLoRes();
   MFBT_API static AwakeTimeStamp Now();
   MFBT_API void operator+=(const AwakeTimeDuration& aOther);

mozglue/misc/TimeStamp.h

@@ -387,6 +387,7 @@ static_assert(sizeof(TimeStampValue) > 8);
  */
 class TimeStamp {
  public:
+  using DurationType = TimeDuration;
   /**
    * Initialize to the "null" moment
    */