Update V8 to r4851 as required by WebKit r61121

Change-Id: Ib01b7c8e38d5b82b254192fc06365aa5b85780c5
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index b0a3fd6..f9b20a4 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -806,7 +806,8 @@
   VerifyObjectField(JSGlobalProxy::kContextOffset);
   // Make sure that this object has no properties, elements.
   CHECK_EQ(0, properties()->length());
-  CHECK_EQ(0, elements()->length());
+  CHECK(HasFastElements());
+  CHECK_EQ(0, FixedArray::cast(elements())->length());
 }