Re-enable an UNREACHABLE in JSObject::GetHeaderSize()

Review URL: https://codereview.chromium.org/1091553002

Cr-Commit-Position: refs/heads/master@{#27908}

src/objects-inl.h

@@ -1979,9 +1979,7 @@ int JSObject::GetHeaderSize() {
     case JS_MESSAGE_OBJECT_TYPE:
       return JSMessageObject::kSize;
     default:
-      // TODO(jkummerow): Re-enable this. Blink currently hits this
-      // from its CustomElementConstructorBuilder.
-      // UNREACHABLE();
+      UNREACHABLE();
       return 0;
   }
 }