Fix build with unbundled zlib

Linux distros like Gentoo have a philosophy of not statically-linking
dependencies.  The reason is not necessarily to save disk space, but
because they like to compile with their own flags and patches applied.
This CL fixes the zlib unbundle for them.

BUG=800977

Change-Id: I129d95a5a4b6c4183666f712609b2e3f0509526e
Reviewed-on: https://chromium-review.googlesource.com/947464
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543178}

third_party/zlib/google/compression_utils.cc

@@ -13,7 +13,12 @@
 #include "base/bit_cast.h"
 #include "base/logging.h"
 #include "base/sys_byteorder.h"
+
+#if defined(USE_SYSTEM_ZLIB)
+#include <zlib.h>
+#else
 #include "third_party/zlib/zlib.h"
+#endif
 
 namespace {