How to reproduce:
1. Open the attached project
2. It has one webm video, select it
3. In Inspector enable transcode, change codec to VP8 (when issue is fixed try decoding to H264 as well) and apply
4. Notice Video and Audio decoding warning
5. Open Sample Scene and play it
Result: audio plays 30FPS video plays 60FPS, because video uses VFR (variable frame rate) and the first video frame happens after only 3.47 seconds

Reproducible: 2017.4.4f1, 2018.2.0b6, 2018.3.0a1
Tested and reproduced:
on Macbook Pro OSX 10.13.4
on Android and iOS devices.

FIXED: after transcoding very lear warning is printed
VFR warning: 63 video frames have a different duration than expected 0.0333333s, ranging from 0.0666332s to 0.1s.
/Users/dominique/UnityProjects/VFR warning/Assets/actual_vfr.mp4 (30FPS) may have variable frame rate (VFR), which is not supported. This may lead to incorrect timing in transcoded clip.