Assembling WebRTC sources fails with error "You have unstaged changes"

After testing the patches in #28725 (moved) and merging them to master I decided to get rid of all by built snowflake related artifacts and deleted out/snowflake, out/go-webrtc and out/webrtc and check if everything is still working.

That does not seem to be the case as I am already getting errors during WebRTC sources assembling now:

Starting build: Fri Dec  7 14:05:23 2018
Bootstrapping cipd client for linux-amd64 from https://chrome-infra-packages.appspot.com/client?platform=linux-amd64&version=git_revision:d2677a4477e59cb7de00f1fb8a00e96b1aaeb927...

src/testing (ERROR)
----------------------------------------
[0:00:02] Started.
[0:00:07] From https://chromium.googlesource.com/chromium/src/testing
[0:00:07]    a5684e6..36a1586  master     -> origin/master
----------------------------------------
Error: 5> 
5> ____ src/testing at 60c665fffe7dc505fdd5d30f9dbcbc50dde1e017
5> 	You have unstaged changes.
5> 	Please commit, stash, or reset.

I wonder if that's a thing for boklm's idea about cleaning up old files, expressed in comment:9:ticket:28725.