Context::Lookup method should support Module variables.

Bug: chromium:717670
Change-Id: I58d3448b9048c7f9dd7ab8b720803b3503cf91ae
Reviewed-on: https://chromium-review.googlesource.com/519389
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45950}