gn format: make sure there's a blank line before comments, except at block start

gn format: make sure there's a blank line before comments, except at block start

With this, gn format //BUILD.gn results in no changes, other than to
make 'deps += [ "a" ]' turn into one line instead of multiline.

(Child of https://codereview.chromium.org/608593006/)

R=brettw@chromium.org
BUG=348474
Committed: https://crrev.com/0abd867e991007102a89979f8316267fb2209cbb
Cr-Commit-Position: refs/heads/master@{#297231}

[brettw, 2014-09-29 16:49:42]

lgtm

[scottmg, 2014-09-29 19:30:57]

Committed patchset #1 (id:1) manually as
0abd867e991007102a89979f8316267fb2209cbb (presubmit successful).

[commit-bot: I haz the power, 2014-09-29 19:30:57]

Patchset 1 (id:??) landed as
https://crrev.com/0abd867e991007102a89979f8316267fb2209cbb
Cr-Commit-Position: refs/heads/master@{#297231}