It's already there... At least for me on 10586.164.
Go to Photos > tap the video to open it > it starts in autoplay > tap the screen > tap the edit (pencil) button > select "Trim"
You get an indicator in your accent color that you can drag. You see a thumbnail for what part of the video you are choosing as you scroll. It leaves a bookmark on your stopping places for reference while you finalize tweaks. Grab the start point and scroll forward. Grab the endpoint and scroll back. There is also a thumbnail for these. Save as copy...