[roheap] Fix unused variable in ReadOnlyHeap::SetUp

Bug: v8:7464
Change-Id: Ieab0007b477894fb8e4c662e584b54cb1e288041
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1680549
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62427}

src/heap/read-only-heap.cc

@@ -36,6 +36,7 @@ void ReadOnlyHeap::SetUp(Isolate* isolate, ReadOnlyDeserializer* des) {
 
   base::CallOnce(&setup_ro_heap_once,
                  [isolate, des, des_checksum, &call_once_ran]() {
+                   USE(des_checksum);
                    shared_ro_heap_ = CreateAndAttachToIsolate(isolate);
                    if (des != nullptr) {
 #ifdef DEBUG