[PA] Add GetSubsequentPageMetadata()

Several call sites in `partition_page.h` use a naked "+ 1" to denote
hopping from one `PartitionPage` to the next. This CL adds a helper
to centralize that logic in a helper function of two variants (const
and non-const args + returns).

Change-Id: I01768c2c4fae13273ce13d1fc06b368776258f8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3829203
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1035017}

base/allocator/partition_allocator/partition_page.h

@@ -419,6 +419,18 @@ PA_ALWAYS_INLINE PartitionPage<thread_safe>* PartitionSuperPageToMetadataArea(
                                                        SystemPageSize());
 }
 
+template <bool thread_safe>
+PA_ALWAYS_INLINE const PartitionPage<thread_safe>* GetSubsequentPageMetadata(
+    const PartitionPage<thread_safe>* page) {
+  return page + 1;
+}
+
+template <bool thread_safe>
+PA_ALWAYS_INLINE PartitionPage<thread_safe>* GetSubsequentPageMetadata(
+    PartitionPage<thread_safe>* page) {
+  return page + 1;
+}
+
 template <bool thread_safe>
 PA_ALWAYS_INLINE PartitionSuperPageExtentEntry<thread_safe>*
 PartitionSuperPageToExtent(uintptr_t super_page) {
@@ -678,23 +690,25 @@ template <bool thread_safe>
 PA_ALWAYS_INLINE void SlotSpanMetadata<thread_safe>::SetRawSize(
     size_t raw_size) {
   PA_DCHECK(CanStoreRawSize());
-  auto* the_next_page = reinterpret_cast<PartitionPage<thread_safe>*>(this) + 1;
+  auto* the_next_page = GetSubsequentPageMetadata(
+      reinterpret_cast<PartitionPage<thread_safe>*>(this));
   the_next_page->subsequent_page_metadata.raw_size = raw_size;
 }
 
 template <bool thread_safe>
 PA_ALWAYS_INLINE size_t SlotSpanMetadata<thread_safe>::GetRawSize() const {
   PA_DCHECK(CanStoreRawSize());
-  auto* the_next_page =
-      reinterpret_cast<const PartitionPage<thread_safe>*>(this) + 1;
+  auto* the_next_page = GetSubsequentPageMetadata(
+      reinterpret_cast<const PartitionPage<thread_safe>*>(this));
   return the_next_page->subsequent_page_metadata.raw_size;
 }
 
 template <bool thread_safe>
 PA_ALWAYS_INLINE PartitionTag*
 SlotSpanMetadata<thread_safe>::DirectMapMTETag() {
   PA_DCHECK(bucket->is_direct_mapped());
-  auto* the_next_page = reinterpret_cast<PartitionPage<thread_safe>*>(this) + 1;
+  auto* the_next_page = GetSubsequentPageMetadata(
+      reinterpret_cast<PartitionPage<thread_safe>*>(this));
   return &the_next_page->subsequent_page_metadata.direct_map_tag;
 }