Gather references to unbound variables where the reference (VariableProxy) is
inside strong mode. Check them against the global object when a script is bound
to a context (during compilation).

This CL only checks unbound variables which are not inside lazy functions - TBD
how do we solve that; alternatives: add developer mode which disables laziness /
do the check whenever lazy functions are really compiled.

BUG=v8:3956
LOG=N

Review URL: https://codereview.chromium.org/1005063002

Cr-Commit-Position: refs/heads/master@{#27422}