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 21548a04973822f5212acdef6f663037b3c8be90..3461de3ef009f18d9549a8de8f18ce831ba2f572 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -1931,9 +1931,10 @@ class PagedSpace : public Space {
   // address denoted by top in allocation_info_.
   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,
+  // Generic fast case allocation function that tries aligned linear allocation
+  // at the address denoted by top in allocation_info_. Writes the aligned
+  // allocation size, which includes the filler size, 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