EmbedderState is forward declared in public header for
EmbedderStateScope. Default std::unique_ptr's destructor needs a fully
defined class. Defining default destructor in implementation file fixes
this.

Bug: chromium:1263871
Change-Id: I8ccc58f56a758927dc5d7a39387188185e7d3827
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3338697
Auto-Submit: Corentin Pescheloche <cpescheloche@fb.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78363}