If you are a multitasker or want to build chromium for different targets without clobbering each other, then perhaps you'll like the gclient-new-workdir.py script located in depot_tools. The script works by creating a new working directory with symlinks pointing to the git database(s) found in your original chromium checkout. You can have as many working directories as you want without the overhead and hassle of cloning chromium multiple times.
gclient-new-workdir.py doesn't support Windows, but you can try https://github.com/joero74/git-new-workdir to do the same thing (needs to be run as admin). For the curious, the script essentially uses mklink /D and other minor tricks to setup the mirrored .git repo.
gclient-new-workdir.py uses symlinks that will not work inside the cros_sdk chroot. If using a local Chromium source for Chromium OS, be sure to use the original working directory.