This was reported to chromium-dev@ (https://groups.google.com/a/chromium.org/d/msg/chromium-dev/JnHke9Qwp90/sFmbSNEZAwAJ) and look like recently closed issue https://bugs.chromium.org/p/chromium/issues/detail?id=806218.

The error reported by users is the following:

  git -c core.deltaBaseCacheLimit=2g clone https://chromium.googlesource.com/external/github.com/material-motion/motion-animator-objc.git 
  fatal: remote error: want cc2b247457dce89c6d377650caf137e77208ae05 not valid
  fatal: The remote end hung up unexpectedly

Marking as P1 as this prevent some user from fetching up Chromium on iOS and contributing.