It's quite easy using the command
avconv from the command line. You do not need to worry about splitting on a keyframe if, as the questioner later specified, you are cutting off an outro (the end of the video). Here's an example:
$ avconv -i videoandoutro.webm -t 00:03:30 -codec: copy videoonly.webm
That would take videoandoutro.webm and create a new file, videoonly.webm and truncate it after exactly 3 minutes and 30 seconds.
Cutting off an intro as well is actually not much harder, but doing that will run into keyframe issues.
$ avconv -ss 00:01:01 -i jossintro.webm -t 00:00:30 -codec: copy oatybar.webm
The above will take a .webm file called "jossintro.webm" and cut a slice starting at around 1 minute, 1 second in. The output is put in "oatybar.webm", which is trimmed to (about) 30 seconds long. The result will actually be slightly longer since the start time will be rounded to the previous keyframe.
[Note that avconv is based on ffmpeg, and so that tool will likely also work, but I have only tested this with avconv.]