Funny enough, I happen to be a video editor, so I feel somewhat obligated to reply to your question. Unfortunately, the industry hasn't yet descended so low that it caters Android tools, which by the sound of it, seem almost specifically for Sup Forums memes. However, if you are looking to create webm files for website design or something along those lines, on a computer, I'd be happy to help, but you'll have to he willing to maybe learn a few things.
First, Download Adobe After Effects, you can get a full 30-day free trial, or even sign up, it's something like 14.95 a month. Congratulations, you're now using the same software as indie film makers, and YouTube creators.
Second, download Adobe Media Encoder. You can find it in the same place as after effects, also free for 30-days.
Third, download a webm extention codec for media Encoder, you can find this online for free, just google it, and download the first result.
HOW TO USE
1. Open after effects, and create a composition, set the parameters. You'll only need to change the duration (maybe make it 00:01:00:00, 1min), resolution (maybe 1920x1080)
2. Import your footage by dragging it into the project area, then drag it into the comp. If you see Black lines around it, it's not to scale, Click right, hit transform, fit to comp.
3. Cut The footage to the area you want, by Click and dragging the tabs on either end of the footage object.
4. Hit ctl-alt-m, that will open the comp in media Encoder, select the webm codec you downloaded before. Now here you can adjust the bitrate, duration, name etc. Select an output then hit render.
Done, thank me later, stop shit posting women.