This fixes an issue where cumulative conditions would fail to evaluate
if the recursed DEP (or custom_var overrides) didn't happen to redeclare
all the variables used by the parent conditions.

TBR=dpranke@google.com, ehmaldonado@google.com

Bug: 825063
Change-Id: Icb53f04928f914dfacc2c3035d01be103d9f8247
Reviewed-on: https://chromium-review.googlesource.com/1000836
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>