Guard third-party heap in V8

Fixes linking issues of MSVC debug build after
https://chromium-review.googlesource.com/c/v8/v8/+/1928860/

Change-Id: I29311103904bef7abdace80edf5f090c756bcbf9
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

v8/src/heap/heap-inl.h

@@ -457,9 +457,11 @@ bool Heap::InOldSpace(Object object) { return old_space_->Contains(object); }
 
 // static
 Heap* Heap::FromWritableHeapObject(HeapObject obj) {
+#ifdef V8_ENABLE_THIRD_PARTY_HEAP
   if (V8_ENABLE_THIRD_PARTY_HEAP_BOOL) {
     return Heap::GetIsolateFromWritableObject(obj)->heap();
   }
+#endif
   BasicMemoryChunk* chunk = BasicMemoryChunk::FromHeapObject(obj);
   // RO_SPACE can be shared between heaps, so we can't use RO_SPACE objects to
   // find a heap. The exception is when the ReadOnlySpace is writeable, during

v8/src/heap/heap.cc

@@ -6589,10 +6589,12 @@ Code Heap::GcSafeFindCodeForInnerPointer(Address inner_pointer) {
   Code code = InstructionStream::TryLookupCode(isolate(), inner_pointer);
   if (!code.is_null()) return code;
 
+#ifdef V8_ENABLE_THIRD_PARTY_HEAP
   if (V8_ENABLE_THIRD_PARTY_HEAP_BOOL) {
     Address start = tp_heap_->GetObjectFromInnerPointer(inner_pointer);
     return GcSafeCastToCode(HeapObject::FromAddress(start), inner_pointer);
   }
+#endif
 
   // Check if the inner pointer points into a large object chunk.
   LargePage* large_page = code_lo_space()->FindPage(inner_pointer);