- represent smis as fake heap numbers
- numbers nodes (both smi and heap numbers) reference a child node whose
  name is "value" and whose entry is the string representation of that
  number

That feature is disabled by default, and can be enabled by passing
captureNumericValue: true when calling HeapProfiler.takeHeapSnapshot

This patch slightly refactors some functions that operate on "essential
objects". We now check that the object is essential before trying to
create the entry. Otherwise, we would end up with smi objects created,
but not referenced anywhere.

Design doc:
https://docs.google.com/document/d/1Qh1zxyn0SS5wzJzitD6ecBJTdFbQkJogSMwxDRsn44o/edit

Change-Id: Ibbe6e79a54c4f9eace72bc0a0ccb622a97698e00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2806747Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Arnaud Renevier <arenevier@fb.com>
Cr-Commit-Position: refs/heads/master@{#73985}