Remove code link from serialization state.

Code::WipeOutHeader now nulls out the next code link to avoid
embedding that address in snapshot.

BUG=
LOG=NO

Review URL: https://codereview.chromium.org/1310503006

Cr-Commit-Position: refs/heads/master@{#30544}

src/objects-inl.h

@@ -6417,6 +6417,7 @@ void Code::WipeOutHeader() {
   if (!READ_FIELD(this, kTypeFeedbackInfoOffset)->IsSmi()) {
     WRITE_FIELD(this, kTypeFeedbackInfoOffset, NULL);
   }
+  WRITE_FIELD(this, kNextCodeLinkOffset, NULL);
 }