I've been helping several people who are new to Chromium, and they've all been bitten by error messages like:

      Illegal include: "path/to/file.h"
    Because of no rule applying.

They've been pretty confused about what this really means and why the include is illegal. I think we should change the error messages in checkdeps_test.py [1] to make it clear that the developer needs to add/update the appropriate DEPS file.

[1] https://cs.chromium.org/chromium/src/buildtools/checkdeps/checkdeps_test.py