Do your work, on a branch here, and commit locally.

  git checkout -b work
  vim third-party-file.cc
  git commit -a -m "work done"

  git cl upload

Do the usual add-reviewers-seek-lgtm dance. Once reviewed, submit your change: 

  git cl land

Done?  Not quite: you need to roll your change into the Chromium tree.

Roll DEPS

Since Chromium mirrors these projects with git, the DEPS file entry for the project has a git hash. So how do you update the DEPS file hash for source code changes that don't really live in git?

  # cd to the main src directory for your checkout
  cd src
  # Create and switch to a new branch
  git new-branch depsroll
  # Run roll-dep giving the path and the desired revision
  # if you omit the revision, it will roll to the current repo HEAD
  roll-dep src/third_party/foo [--roll-to=SVN_REVISION_NUMBER/GIT_COMMIT_HASH]

Method #2: Manual inspection (patch the hash into the DEPS file by hand).

# cd to the main src directory for your checkout
pushd src
# Create and switch to a new branch
git new-branch depsroll
# cd to the directory listed in the DEPS file (e.g., src/third_party/cld_2/src for 'cld2')
pushd third_party/foo/bar
# Fetch (but do not pull) the latest revisions from Chromium's git mirror of the remote repository
git fetch origin
# Use 'git log' to list the commits in the mirror and find the hash of the one you need
git log origin
# Go back to src/
popd
# Patch the revision hash into the DEPS file by hand.

  git commit -a -m "roll third_party/whatever deps because ..."
  git cl upload

Adding new repositories to DEPS

Ensure the repository in question is mirrored at chromium.googlesource.com. We do this to avoid swamping upstream hosts with loads of traffic from our developers and bot fleet. To mirror a repository, please file an infrastructure ticket.