Clean up aligned allocation code in preparation for SIMD alignments.

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index b1a8c5135d9698dfd17c6a6b57ba80ec8b5dc75f..b0f80a2c4632677b52c26ebaaa790ff632bc0b67 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -1938,8 +1938,9 @@ class PagedSpace : public Space {
   inline HeapObject* AllocateLinearly(int size_in_bytes);
 
   // Generic fast case allocation function that tries double aligned linear
-  // allocation at the address denoted by top in allocation_info_.
-  inline HeapObject* AllocateLinearlyAligned(int size_in_bytes,
+  // allocation at the address denoted by top in allocation_info_. Writes actual
+  // allocation size, including alignment fill, to size_in_bytes.
+  inline HeapObject* AllocateLinearlyAligned(int* size_in_bytes,
                                              AllocationAlignment alignment);
 
   // If sweeping is still in progress try to sweep unswept pages. If that is