Teach PRESUBMIT.py how to run gn check for most of the mojo repo

Teach PRESUBMIT.py how to run gn check for most of the mojo repo

Almost all of the mojo-forked code now passes gn check.
I used:
https://github.com/domokit/mojo/wiki/Directory-guide
for the list of mojo-forked directories.

gn check //mojo/* was 152 errors and is now down to 52.

R=jamesr@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/dea154634ceba9cb855619259aa643857b9995ac

[jamesr, 2014-11-07 01:29:54]

I think the //mojo/edk/js changes may conflict with Hans' patch, so hold off on
that, but the rest lgtm.

[eseidel, 2014-11-07 17:18:36]

Committed patchset #3 (id:40001) manually as
dea154634ceba9cb855619259aa643857b9995ac (presubmit successful).