PA: Remove DCHECK from AllocFlagsNoHooks->PartitionRefCountPointer path

If GigaCage isn't enabled, this DCHECK will get triggered:
AllocFlagsNoHooks->PartitionRefCountPointer->DCheckGetSlotOffsetIsZero->
->PartitionAllocGetSlotOffset->
->DCheckIfManagedByPartitionAllocNormalBuckets

GigaCage isn't needed to set up PartitionRefCount. It's only needed to
easily detect from BackupRefPtr code if PartitionRefCount is available,
but this won't be an issue because BackupRefPtr is disabled when
GigaCage isn't needed.

Bug: 1073933
Change-Id: Iaea2781d207e9534293a1b3dd57dd6c42a422ed7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620922Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842036}

base/allocator/partition_allocator/partition_ref_count.h

@@ -108,6 +108,11 @@ ALWAYS_INLINE PartitionRefCount* PartitionRefCountPointer(void* slot_start) {
   return reinterpret_cast<PartitionRefCount*>(slot_start);
 }
 
+ALWAYS_INLINE PartitionRefCount* PartitionRefCountPointerNoDCheck(
+    void* slot_start) {
+  return reinterpret_cast<PartitionRefCount*>(slot_start);
+}
+
 #else  // ENABLE_REF_COUNTER_FOR_BACKUP_REF_PTR
 
 static constexpr size_t kInSlotRefCountBufferSize = 0;

base/allocator/partition_allocator/partition_root.h

@@ -1234,7 +1234,7 @@ ALWAYS_INLINE void* PartitionRoot<thread_safe>::AllocFlagsNoHooks(
   bool is_direct_mapped = raw_size > kMaxBucketed;
   if (allow_extras && !is_direct_mapped) {
 #if ENABLE_REF_COUNT_FOR_BACKUP_REF_PTR
-    new (internal::PartitionRefCountPointer(slot_start))
+    new (internal::PartitionRefCountPointerNoDCheck(slot_start))
         internal::PartitionRefCount();
 #endif  // ENABLE_REF_COUNT_FOR_BACKUP_REF_PTR
   }