Fix an instance of -Wunused-but-set-variable.

Bug: chromium:1203071
Change-Id: I09724552731594ddf18358d29cdc243cb696652d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3053617Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75929}

test/cctest/heap/test-lab.cc

@@ -95,11 +95,8 @@ TEST(SimpleAllocate) {
     LocalAllocationBuffer lab =
         LocalAllocationBuffer::FromResult(heap, lab_backing_store, kLabSize);
     CHECK(lab.IsValid());
-    intptr_t sum = 0;
     for (auto size : sizes) {
-      if (AllocateFromLab(heap, &lab, size)) {
-        sum += size;
-      }
+      AllocateFromLab(heap, &lab, size);
     }
   }
   VerifyIterable(base, limit, expected_sizes);