Add kHeaderSize constant to SeqString.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index adfb74a4efeabf9868a7be678c15dcb6b4603141..ddbcd624d497ec121e11d9411dcea2d84a9b28e3 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -437,6 +437,7 @@ MemoryChunk* MemoryChunk::Initialize(Heap* heap,
   chunk->SetFlag(WAS_SWEPT_PRECISELY);
 
   ASSERT(OFFSET_OF(MemoryChunk, flags_) == kFlagsOffset);
+  ASSERT(OFFSET_OF(MemoryChunk, live_byte_count_) == kLiveBytesOffset);
 
   if (executable == EXECUTABLE) chunk->SetFlag(IS_EXECUTABLE);
 
@@ -822,8 +823,7 @@ void PagedSpace::Verify(ObjectVisitor* visitor) {
       ASSERT(object->address() + size <= top);
       end_of_previous_object = object->address() + size;
     }
-    // TODO(1672): Page live bytes are off for some tests.
-    // CHECK_LE(black_size, page->LiveBytes());
+    CHECK_LE(black_size, page->LiveBytes());
   }
 }
 #endif