Clean up Stats class used in VideoProcessor tests.

* Reorder and rename members of FrameStatistic.
* Rename corresponding temp variables used in the summary calculations.
* Make Stats class less structy.
* Try to enforce consistent frame numbers at all times.
* Remove VideoProcessor::FrameInfo struct and use the Stats class
  for keeping track of encode/decode start times, etc.


Cr-Original-Commit-Position: refs/heads/master@{#19703}
Cr-Mirrored-Commit: 8935d97f169d0c85ea04d2f370964f283783cd79
7 files changed