Also define classes NativeContext and InternalizedString. Those object
kinds were already part of our Object hierarchy but didn't have their
own class, which was inconvenient.

R=jarin@chromium.org, mslekova@chromium.org

Bug: v8:7790
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ic443a2b2e34afc89bc924e845d995e3f287a2535
Reviewed-on: https://chromium-review.googlesource.com/1185592Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55402}