Fix Linux unbundled zlib.gyp's duplicate target.

net/spdy/spdy_framer.cc

Index: net/spdy/spdy_framer.cc
diff --git a/net/spdy/spdy_framer.cc b/net/spdy/spdy_framer.cc
index faf49b407b1ab329616e1019d30d3ce49724b220..de41774a0fdb83343dfd13dafc270120beced275 100644
--- a/net/spdy/spdy_framer.cc
+++ b/net/spdy/spdy_framer.cc
@@ -39,6 +39,7 @@ uLong CalculateDictionaryId(const char* dictionary,
                  dictionary_size);
 }
 
+#if !defined(USE_SYSTEM_ZLIB)
 // Check to see if the name and value of a cookie are both empty.
 bool IsCookieEmpty(const base::StringPiece& cookie) {
   if (cookie.size() == 0) {
@@ -57,6 +58,7 @@ bool IsCookieEmpty(const base::StringPiece& cookie) {
   }
   return (pos == 0) && ((cookie.size() - value_start) == 0);
 }
+#endif  // !defined(USE_SYSTEM_ZLIB)
 
 // Pack parent stream ID and exclusive flag into the format used by HTTP/2
 // headers and priority frames.