Assuming you have a local chromium checkout you've been building natively, and you want to be able to hack in it and build it for a CrOS device, see instructions below.
Following http://dev.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser, a one-time setup step is:
$ cros_sdk --enter --chrome_root=.../path/to/chrome/dir [ --chroot=../../which_chroot ]
(where the chrome/dir part is the parent of src/).
Subsequent cros_sdk invocations will have the chrome source directory mounted at ~/chrome_root/src inside the chroot.
Make sure you've run build_packages and build_image at least one successfully in the repo in question, or it's likely you'll be missing packages/binaries/includes/libs needed to build chrome! See CrOS dev guide for details on those commands.
Inside the parent of the chrome src/ dir, I have two scripts: "ninja" and "goma-ninja" (attached to this page):
Having these scripts in the parent of the src/ dir means they're available both inside the chroot and outside it, so regardless of where I am, I can always say "../goma-ninja chrome" for example and the right thing happens.
Finally, I have a file named bashrc-tail which I source inside the chroot with:
at the bottom of my chroot's ~/.bashrc. This defines two shell functions ("lumpy" and "daisy") which set up environment variables facilitating building for different CrOS boards, as well as shell functions that help run chrome and other ninja outputs on the boards, without having to explicitly copy things around, by using sshfs. After a build like ../goma-ninja chrome from above is done, I'll run initcrosdevice which will ssh into the device (note: make sure to update the crosip() functions in your copy of bashrc-tail to point to the right IP addresses of your boards, as well as the username/hostname/path in the definition of initcrosdevice!) and echo a helpful message useful for copy/paste to sshfs-mount the ninja output directory on the board. At that point I can run a unittest like:
DISPLAY=:0 ./Debug/video_decode_accelerator_unittestor run a full chrome. Beware that running chrome on a board usually requires a pile of switches (depending on what features are launched on CrOS at a given time, and the state of the drivers on any given board at any given time), which you are best off extracting from the "real" chrome running on the device. I usually keep these switches in a script at out_daisy/Debug/go, so that it's available via the sshfs mount above and I can just run it on the device as ./Debug/go.