This brings V8's behavior in line with both the spec and with
other engines.

This also fixes the (now-incorrect) DCHECK in BytecodeGenerator relating
to the delete operator's application to a VariableProxy.

Bug: v8:6697, v8:6721
Change-Id: I413c02af235b0bb652eb4c5d5c971e2cf80e0906
Reviewed-on: https://chromium-review.googlesource.com/703894Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48346}