Use gn check_targets and gn gen --check instead of presubmit

Use gn check_targets and gn gen --check instead of presubmit

gn has the ability to verify that 'gn check' passes for a set of targets
during generation. Use this instead of a presubmit to avoid having to
run gn multiple times and to get coverage on every run instead of just
on presubmits.

R=eseidel@google.com, eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/5f36d3794485583400bd8b8c38bc114ba367abae

[eseidel1, 2015-01-08 19:10:45]

eseidel@google.com changed reviewers:
+ eseidel@google.com

[eseidel1, 2015-01-08 19:10:46]

lgtm

Does everythign pass gn check now?  I don't see how your new code filters to
only the targets we know to pass gn check

[jamesr, 2015-01-08 19:14:25]

https://codereview.chromium.org/821103003/diff/1/.gn
File .gn (right):

https://codereview.chromium.org/821103003/diff/1/.gn#newcode14
.gn:14: check_targets = [
this code filters

[jamesr, 2015-01-08 21:04:01]

Committed patchset #1 (id:1) manually as
5f36d3794485583400bd8b8c38bc114ba367abae (presubmit successful).