[heap][cleanup] Fix tracking two background scope symbols

BACKGROUND_UNPARK and BACKGROUND_SAFEPOINT were introduced in
crrev.com/c/2704075 but the LAST_GENERAL_BACKGROUND_SCOPE was
not updated, so they were not tracked properly by GCTracer.

Bug: v8:12425
Change-Id: Ibbd7f5410088c45454b6e22af4e038e8ef0dd7fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3650598Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80613}

src/heap/gc-tracer.h

@@ -82,7 +82,7 @@ class V8_EXPORT_PRIVATE GCTracer {
       NUMBER_OF_INCREMENTAL_SCOPES =
           LAST_INCREMENTAL_SCOPE - FIRST_INCREMENTAL_SCOPE + 1,
       FIRST_GENERAL_BACKGROUND_SCOPE = BACKGROUND_YOUNG_ARRAY_BUFFER_SWEEP,
-      LAST_GENERAL_BACKGROUND_SCOPE = BACKGROUND_UNMAPPER,
+      LAST_GENERAL_BACKGROUND_SCOPE = BACKGROUND_SAFEPOINT,
       FIRST_MC_BACKGROUND_SCOPE = MC_BACKGROUND_EVACUATE_COPY,
       LAST_MC_BACKGROUND_SCOPE = MC_BACKGROUND_SWEEPING,
       FIRST_TOP_MC_SCOPE = MC_CLEAR,