Revert "Remove failure tag.".

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index a3cc737784f1947ba430d6b17e39b989349bc241..efed18ec6d4778c26ef7bd1fd1067e68230b43ff 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -312,11 +312,20 @@ class MemoryChunk {
   }
 
   Space* owner() const {
-    return owner_;
+    if ((reinterpret_cast<intptr_t>(owner_) & kFailureTagMask) ==
+        kFailureTag) {
+      return reinterpret_cast<Space*>(reinterpret_cast<intptr_t>(owner_) -
+                                      kFailureTag);
+    } else {
+      return NULL;
+    }
   }
 
   void set_owner(Space* space) {
-    owner_ = space;
+    ASSERT((reinterpret_cast<intptr_t>(space) & kFailureTagMask) == 0);
+    owner_ = reinterpret_cast<Address>(space) + kFailureTag;
+    ASSERT((reinterpret_cast<intptr_t>(owner_) & kFailureTagMask) ==
+           kFailureTag);
   }
 
   base::VirtualMemory* reserved_memory() {
@@ -682,7 +691,10 @@ class MemoryChunk {
 
   // If the chunk needs to remember its memory reservation, it is stored here.
   base::VirtualMemory reservation_;
-  Space* owner_;
+  // The identity of the owning space.  This is tagged as a failure pointer, but
+  // no failure can be in an object, so this can be distinguished from any entry
+  // in a fixed array.
+  Address owner_;
   Heap* heap_;
   // Used by the store buffer to keep track of which pages to mark scan-on-
   // scavenge.