Allow paths to be resolved in the GN args file

Allow paths to be resolved in the GN args file

This uses the real settings object which contains the location of the
source root for resolving "//..." paths. The use-case is to allow
imports to be processed in the args file, which in turn allows one to assign
a set of build arguments by reference.

Committed: https://crrev.com/c2f498dd8cb63c49663b5f80aabf808b4fd1d1c4
Cr-Commit-Position: refs/heads/master@{#327131}

[brettw, 2015-04-27 19:22:21]

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

[scottmg, 2015-04-27 20:17:48]

lgtm

[brettw, 2015-04-27 20:25:52]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2015-04-27 20:26:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1104313002/1

[commit-bot: I haz the power, 2015-04-27 21:31:11]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-27 21:32:13]

Patchset 1 (id:??) landed as
https://crrev.com/c2f498dd8cb63c49663b5f80aabf808b4fd1d1c4
Cr-Commit-Position: refs/heads/master@{#327131}