Change the existing uses of the harmony-class-fields flag to
harmony-public-fields so that we can stage this separately
from the upcoming harmony-private-fields to get some
clusterfuzz coverage.

Bug: v8:5367
Change-Id: I76cdefa4faf34eae73d3a5f6d6089cf75677732a
Reviewed-on: https://chromium-review.googlesource.com/792940
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49661}