When preparsing top-level functions in a module, we didn't track
unresolved variables. Consequently, "this" ended up referencing
the global "this", which has the wrong value (in a module "this"
is supposed to be the undefined value).

This patch fixes that. This also lets us stop forcing context
allocation of all variables in module scopes, which the patch
takes care of as well.

Bug: chromium:791334
Change-Id: Ifac1f1adc033f3facfb3d29dd4bca32ee27bffcf
Reviewed-on: https://chromium-review.googlesource.com/808938Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50025}