Also move some code from separated function into
JSDateTimeFormat::Initialize as suggested in review of previous CL.

Bug: v8:8066
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I6be61482759aa54890e8f9d86483436a1abcde87
Reviewed-on: https://chromium-review.googlesource.com/c/1252882
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56459}