[PartitionAlloc] Finish transition to SlotSpanMetadata

Continuation of crrev.com/c/2466007

Change-Id: Idde6926a2dc186fe843052b5f312c16576a641fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482563
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818341}

base/allocator/partition_allocator/partition_bucket.cc

@@ -116,7 +116,7 @@ PartitionDirectMap(PartitionRoot<thread_safe>* root, int flags, size_t raw_size)
   map_extent->prev_extent = nullptr;
   root->direct_map_list = map_extent;
 
-  return reinterpret_cast<SlotSpanMetadata<thread_safe>*>(page);
+  return &page->slot_span_metadata;
 }
 
 }  // namespace

base/allocator/partition_allocator/partition_page.h

@@ -256,8 +256,8 @@ ALWAYS_INLINE bool IsWithinSuperPagePayload(char* ptr, bool with_pcscan) {
 template <bool thread_safe>
 ALWAYS_INLINE SlotSpanMetadata<thread_safe>*
 SlotSpanMetadata<thread_safe>::FromPointerNoAlignmentCheck(void* ptr) {
-  return reinterpret_cast<SlotSpanMetadata*>(
-      PartitionPage<thread_safe>::FromPointerNoAlignmentCheck(ptr));
+  return &PartitionPage<thread_safe>::FromPointerNoAlignmentCheck(ptr)
+              ->slot_span_metadata;
 }
 
 // See the comment for |FromPointer|.