[libfuzzer] Add expat_xml_parse_fuzzer for third_party/expat.

third_party/expat/BUILD.gn

Index: third_party/expat/BUILD.gn
diff --git a/third_party/expat/BUILD.gn b/third_party/expat/BUILD.gn
index 533296e55c68f51d7ebb56d322c5565fbb6925c6..e70f31be067f4c0562daa86f364b1048e843d29f 100644
--- a/third_party/expat/BUILD.gn
+++ b/third_party/expat/BUILD.gn
@@ -3,6 +3,7 @@
 # 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) {
@@ -37,3 +38,13 @@ if (is_linux && !is_chromecast) {
     }
   }
 }
+
+fuzzer_test("expat_xml_parse_fuzzer") {
+  sources = [
+    "fuzz/expat_xml_parse_fuzzer.cc",
+  ]
+  deps = [
+    ":expat",
+  ]
+  dict = "//testing/libfuzzer/fuzzers/dicts/xml.dict"
+}