[libfuzzer] use source-built expat parser

third_party/expat/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chromecast_build.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
-# Chromecast doesn't ship expat as a system library
-if (is_linux && !is_chromecast) {
+# On Linux, we implicitly already depend on expat via fontconfig;
+# let's not pull it in twice.
+# Chromecast doesn't ship expat as a system library.
+# Libfuzzer needs to build library from sources.
+if (is_linux && !is_chromecast && !use_libfuzzer) {
   config("expat_config") {
     libs = [ "expat" ]
   }
 
   group("expat") {
     public_configs = [ ":expat_config" ]
   }
 } else {
   config("expat_config") {
     include_dirs = [ "files/lib" ]
@@ skipping 21 matching lines @@
 
 fuzzer_test("expat_xml_parse_fuzzer") {
   sources = [
     "fuzz/expat_xml_parse_fuzzer.cc",
   ]
   deps = [
     ":expat",
   ]
   dict = "//testing/libfuzzer/fuzzers/dicts/xml.dict"
 }