Add partitionAllocActualSize() for predicting actual size of allocation

Source/wtf/PartitionAlloc.cpp

Index: Source/wtf/PartitionAlloc.cpp
diff --git a/Source/wtf/PartitionAlloc.cpp b/Source/wtf/PartitionAlloc.cpp
index a481e40043ae957ec94b2d59253f2b420dbfccea..a06506c66afabef9ce754ad516702e192308aff7 100644
--- a/Source/wtf/PartitionAlloc.cpp
+++ b/Source/wtf/PartitionAlloc.cpp
@@ -526,8 +526,8 @@ static ALWAYS_INLINE bool partitionSetNewActivePage(PartitionPage* page)
 
 static ALWAYS_INLINE void* partitionDirectMap(PartitionRootBase* root, int flags, size_t size)
 {
-    size += kSystemPageOffsetMask;
-    size &= kSystemPageBaseMask;
+    size = partitionDirectMapSize(size);
+
     // Because we need to fake looking like a super page, We need to allocate
     // a bunch of system pages more than "size":
     // - The first few system pages are the partition page in which the super