Class fields needs to be initialized after `this` is bound, as per the
new spec change:
https://github.com/tc39/proposal-class-fields/pull/92

This CL moves the initialization of `this` from parser desugaring to
the bytecode generator.

Bug: v8:7647
Change-Id: I20f749403e5a4d2f06a39726cf39012ceb541987
Reviewed-on: https://chromium-review.googlesource.com/1014383Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52646}