In order to implement CodeEntry deallocation when profiles are stopped,
we need to be able to effectively deallocate strings. Introduce a simple
imperative refcounting API using the existing HashMap slots for
StringsStorage to enable this.

Design doc: https://docs.google.com/document/d/1OTwlBnAMXZEaOICtuz16c01QnkPPdqHBoHpfGwnk5SY/edit

Bug: chromium:956688
Change-Id: Iaa1142925f40aa66c064d011b2a0630de72037fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2121575Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#66911}