Fix PartitionAllocTest.PartialPageFreelists for ENABLE_REF_COUNT_FOR_BACKUP_REF_PTR debug builds

Bug: 1080832
Change-Id: Ifb7f704416ec9e993f1c180a3a72a76b7ffb2452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578417Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834624}

base/allocator/partition_allocator/partition_alloc_unittest.cc

@@ -1222,8 +1222,7 @@ TEST_F(PartitionAllocTest, MAYBE_PartialPageFreelists) {
       (slot_span->bucket->num_system_pages_per_slot_span * SystemPageSize()) /
       (very_small_size + kExtraAllocSize);
   first_slot_span_slots =
-      (SystemPageSize() + very_small_size + kExtraAllocSize - 1) /
-      (very_small_size + kExtraAllocSize);
+      SystemPageSize() / (very_small_size + kExtraAllocSize);
   EXPECT_EQ(total_slots - first_slot_span_slots,
             slot_span->num_unprovisioned_slots);