Minor improvements to VideoProcessor and corresponding test.

- Make all overridden methods of VideoProcessorImpl public,
  in preparation of the removal of the VideoProcessor interface.
- Place corresponding method definitions in correct order
  in .cc file.
- Harmonize the stdout printing.
- Make timestamp calculations adhere to set frame rate.

Except for the last bullet, these changes should not lead to
different functionality.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/2995513002
Cr-Original-Commit-Position: refs/heads/master@{#19254}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bea36fdee86f7c5eb80dc9d1032b61af01e92ccb