Oilpan: Update a comment for the WTF_ANNOTATE_LEAKING_OBJECT_PTR macro in PageMemory

Source/platform/heap/Heap.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 259 matching lines @@
         RELEASE_ASSERT(res);
         return storage;
 #endif
     }
 
 private:
     PageMemory(const MemoryRegion& reserved, const MemoryRegion& writable)
         : m_reserved(reserved)
         , m_writable(writable)
     {
-        // Leak Sanitizer runs before the shutdown sequence runs, and thus PageMemory objects
-        // are reported as leaking. So we annotate to let the Leak Sanitizer ignore PageMemory objects.
+        // This annotation is for letting the LeakSanitizer ignore PageMemory objects.
+        //
+        // - The LeakSanitizer runs before the shutdown sequence and reports unreachable memory blocks.
+        // - The LeakSanitizer only recognizes memory blocks allocated through malloc/new,
+        //   and we need special handling for mapped regions.
+        // - The PageMemory object is only referenced by a HeapPage<Header> object, which is
+        //   located inside the mapped region, which is not released until the shutdown sequence.
+        //
+        // Given the above, we need to explicitly annotate that the LeakSanitizer should ignore
+        // PageMemory objects.
         WTF_ANNOTATE_LEAKING_OBJECT_PTR(this);
 
         ASSERT(reserved.contains(writable));
     }
 
     MemoryRegion m_reserved;
     MemoryRegion m_writable;
 };
 
 class GCScope {
@@ skipping 1384 matching lines @@
 template class HeapPage<FinalizedHeapObjectHeader>;
 template class HeapPage<HeapObjectHeader>;
 template class ThreadHeap<FinalizedHeapObjectHeader>;
 template class ThreadHeap<HeapObjectHeader>;
 
 Visitor* Heap::s_markingVisitor;
 CallbackStack* Heap::s_markingStack;
 CallbackStack* Heap::s_weakCallbackStack;
 bool Heap::s_shutdownCalled = false;
 }