gn format: Add --dry-run flag for PRESUBMIT

gn format: Add --dry-run flag for PRESUBMIT

`gn format --dry-run some/BUILD.gn` sets exit code to 0 for "success,
unchanged", 1 for "failure", and 2 for "success but different than on
disk". This is to be able to add a PRESUBMIT.py check in the future and
matches the behaviour of the same-named flag for git cl format.

See also https://codereview.chromium.org/779513005/.

R=brettw@chromium.org
BUG=348474
Committed: https://crrev.com/f5d95bf7fb5ee07c82e62cb573af67b195284bc3
Cr-Commit-Position: refs/heads/master@{#306940}

[brettw, 2014-12-04 23:51:26]

lgtm

[scottmg, 2014-12-05 00:02:11]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-12-05 00:02:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/779883002/1

[commit-bot: I haz the power, 2014-12-05 00:05:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-05 00:06:32]

Patchset 1 (id:??) landed as
https://crrev.com/f5d95bf7fb5ee07c82e62cb573af67b195284bc3
Cr-Commit-Position: refs/heads/master@{#306940}