Rename failure tag to page owner tag and add proper description.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index bddd4ef056c0538447bc7982d046dc789b142755..5959750a72a549aaaded258f49a9b8b306268f89 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -216,11 +216,15 @@ const int kCodeAlignmentBits = 5;
 const intptr_t kCodeAlignment = 1 << kCodeAlignmentBits;
 const intptr_t kCodeAlignmentMask = kCodeAlignment - 1;
 
-// Tag information for Failure.
-// TODO(yangguo): remove this from space owner calculation.
-const int kFailureTag = 3;
-const int kFailureTagSize = 2;
-const intptr_t kFailureTagMask = (1 << kFailureTagSize) - 1;
+// The owner field of a page is tagged with the page header tag. We need that
+// to find out if a slot is part of a large object. If we mask out the lower
+// 0xfffff bits (1M pages), go to the owner offset, and see that this field
+// is tagged with the page header tag, we can just look up the owner.
+// Otherwise, we know that we are somewhere (not within the first 1M) in a
+// large object.
+const int kPageHeaderTag = 3;
+const int kPageHeaderTagSize = 2;
+const intptr_t kPageHeaderTagMask = (1 << kPageHeaderTagSize) - 1;
 
 
 // Zap-value: The value used for zapping dead objects.