Remove GC metadata of code object before serializing.

Follow up on discussion of https://codereview.chromium.org/1310503006/

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1313953008

Cr-Commit-Position: refs/heads/master@{#30562}

src/objects-inl.h

@@ -6426,6 +6426,7 @@ void Code::WipeOutHeader() {
     WRITE_FIELD(this, kTypeFeedbackInfoOffset, NULL);
   }
   WRITE_FIELD(this, kNextCodeLinkOffset, NULL);
+  WRITE_FIELD(this, kGCMetadataOffset, NULL);
 }