Make instance_size immediately useful for all fixed size objects.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 0a971283f2ed5ed0abf94ea9f5175caebbf1d360..91aba269eb8491faa2485419f9759455457b5c70 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -640,8 +640,9 @@ void Map::MapPrint() {
 void Map::MapVerify() {
   ASSERT(!Heap::InNewSpace(this));
   ASSERT(FIRST_TYPE <= instance_type() && instance_type() <= LAST_TYPE);
-  ASSERT(kPointerSize <= instance_size()
-         && instance_size() < Heap::Capacity());
+  ASSERT(instance_size() == kVariableSizeSentinel ||
+         (kPointerSize <= instance_size() &&
+          instance_size() < Heap::Capacity()));
   VerifyHeapPointer(prototype());
   VerifyHeapPointer(instance_descriptors());
 }