In order to ensure that Isolate::New()/Isolate::Delete() are the bottlenecks
this CL also makes the Isolate class final.

Bug: v8:8182
Change-Id: I6bb170363a1210f66d63f4bcc46ea06fb5000d50
Reviewed-on: https://chromium-review.googlesource.com/c/1301481
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57041}