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

Review URL: https://codereview.chromium.org/566963002

Cr-Commit-Position: refs/heads/master@{#294634}

native_client_sdk/src/build_tools/buildbot_common.py

@@ -80,6 +80,7 @@ def GetWindowsEnvironment():
       self.env[key] = value
 
   context = FakeContext()
+  context['gyp_vars'] = []
   buildbot_standard.SetupWindowsEnvironment(context)
 
   # buildbot_standard.SetupWindowsEnvironment adds the directory which contains