Converting Webm to MP4 using latest avconv or ffmpeg

Ok, there is no ffmpeg here since its so old and we have all been using the latest avconv. However, there are so many legacy documentation about ffmpeg conversion and webm that leaves me searching up and downs for a solutions for my problems. Although in the end, it wasn't my problem at all but i manage to resolve it somehow so writing it out might helps someone out there too.

Problems converting webm to mp4

if you are facing problems converting webm (whammy) from webrtc of Chrome using older version of ffmpeg or avconv. You are not alone. You may face the following problems