The is_public name is misleading now that we have private
symbols. It's only used to know if the symbol is stored in the global
symbol registry.

This patch renames it to a more suitable name.

Change-Id: I78f31cb5438416d07b78a9038c3526c9dfb96c6e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1738849Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63092}