GN: Fix dependency output file handling.

GN: Fix dependency output file handling.

A copy and paste error meant it used the link output file for dependency tracking. This is valid but inefficient, which was why it wasn't caught. Added a new test for this case

R=scottmg@chromium.org

Committed: https://crrev.com/4fab8a0b3d7e2ae3db541795783b41911ad8c711
Cr-Commit-Position: refs/heads/master@{#295985}

[brettw, 2014-09-19 22:54:49]

brettw@chromium.org changed reviewers:
+ scottmg@chromium.org

[scottmg, 2014-09-19 23:02:39]

LGTM

[brettw, 2014-09-22 16:31:39]

Committed patchset #1 (id:1) to pending queue manually as 96be36d (presubmit
successful).

[commit-bot: I haz the power, 2014-09-22 16:32:35]

Patchset 1 (id:??) landed as
https://crrev.com/4fab8a0b3d7e2ae3db541795783b41911ad8c711
Cr-Commit-Position: refs/heads/master@{#295985}