[heap] Refactor Heap::PerformGarbageCollection

This ensures that PerformGarbageCollection runs completely within a
LocalHeap safepoint. External prologues and epilogues that may trigger
GC and run JS are moved outside.

Bug: v8:10315

Change-Id: I5c0081f0791ba5d27152c119a2a0d454056656d3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190756
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67736}

src/heap/heap.h

@@ -1635,10 +1635,9 @@ class Heap {
   // over all objects.  May cause a GC.
   void MakeHeapIterable();
 
-  // Performs garbage collection
-  // Returns whether there is a chance another major GC could
-  // collect more garbage.
-  bool PerformGarbageCollection(
+  // Performs garbage collection in a safepoint.
+  // Returns the number of freed global handles.
+  size_t PerformGarbageCollection(
       GarbageCollector collector,
       const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);