BytecodeGenerator previously assumed that any UNALLOCATED variable
must be a global object property, but that's incorrect for global
lexical variables declared in a different script.

This patch fixes the behavior by always falling back to the runtime
to deal with deleting UNALLOCATED variables. This is sub-optimal,
but should be correct, and it's unclear if speed is important for
this case.

Bug: v8:6733
Change-Id: I83c2a0b6e30e5e5f4c79bfe14ebf196529816c71
Reviewed-on: https://chromium-review.googlesource.com/627636Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47554}