How to repro a build from a waterfall.

Reproducing a builder's build can generally be done by following the instructions located under the 'run_recipe' link on the build page itself.

In particular, you need to install depot_tools, and then get a build checkout:

$ cd <empty dir>
$ # One of:
$ fetch infra           # external user
$ fetch infra_internal  # internal user
$ cd build

And then navigate to a build that you want to repo. Look for the build step which looks like:

==setup_build== setup_build
running recipe: "<RECIPE_NAME>" ( 0 secs )
*   ==stdio==
*   ==run_recipe==

Then click the 'run_recipe' link and you'll get some instructions on how to repro the build.