This change adds new functions to BackgroundCompileTask which closely
match those in BackgroundDeserializeTask. These functions allow a caller
to manage background merging of newly compiled content into an existing
Script from the Isolate compilation cache. These functions are not yet
exposed via the API; instead, StressBackgroundCompileThread uses them to
increase test coverage of the merging logic.

Bug: v8:12808
Change-Id: I4d2f429164223785169fe447ce2bdd8beaee00d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793959
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82121}