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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}