If you need it faster, download FFmpeg and exec the following command:
ffmpeg -i input_video.webm -vf "scale=ceil(iw/2)*2:ceil(ih/2)*2" -c:v libx264 -c:a aac -strict experimental output_video.mp4
The scale filter allows to transcode any size to libx264 which requires both dimensions to be divisible by 2.