For measuring the Dart thread, we care about thread duration (tdur) rather than
wall duration (dur) because we don't want to count the time when the Dart
thread is descheduled (e.g., in preference to the raster thread).

Prior to this change, these benchmarks were mostly measuring whether the OS
decided to finish the Dart thread's time slice or hand over the CPU to the
raster thread to complete the visual part of the frame. Now we actually measure
the work done on the Dart thread.