Add gyp_vars to fake context in sdk build script.

Add gyp_vars to fake context in sdk build script.

We reuse SetupWindowEnvironment from the nacl build, in the sdk build.
To do this we inject a fake build context.
Previously windows in particular just needed an empty mutable context.
Now it assumes gyp_vars is set to a list of some kind.
Initializing 'gyp_vars' to unblock a deps roll.

BUG=None
TEST=trybots
R=binji@chromium.org

Committed: https://crrev.com/5831c275cccc77e5eb9074377bc69c3d795f0778
Cr-Commit-Position: refs/heads/master@{#294634}

[binji, 2014-09-12 17:18:20]

lgtm

[bradn, 2014-09-12 17:18:27]

The CQ bit was checked by bradnelson@google.com

[commit-bot: I haz the power, 2014-09-12 17:19:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/566963002/1

[commit-bot: I haz the power, 2014-09-12 19:35:11]

Committed patchset #1 (id:1) as 44ee6b5e172e1819cecb4043dd13e0f1b854cbd7

[commit-bot: I haz the power, 2014-09-12 19:38:06]

Patchset 1 (id:??) landed as
https://crrev.com/5831c275cccc77e5eb9074377bc69c3d795f0778
Cr-Commit-Position: refs/heads/master@{#294634}