Add a fuzzer for SfntlyWrapper::SubsetFont().

third_party/sfntly/BUILD.gn

Index: third_party/sfntly/BUILD.gn
diff --git a/third_party/sfntly/BUILD.gn b/third_party/sfntly/BUILD.gn
index 621b463197d7a48683bcfde2e3523ee0721e9ae2..b801367337b0c771a69bb90d6a415d3a34c82117 100644
--- a/third_party/sfntly/BUILD.gn
+++ b/third_party/sfntly/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//testing/libfuzzer/fuzzer_test.gni")
+
 declare_args() {
   # Flip to true to build sfntly sample programs.
   build_sfntly_samples = false
@@ -158,6 +160,17 @@ static_library("sfntly") {
   ]
 }
 
+fuzzer_test("sfntly_fuzzer") {
+  sources = [
+    "fuzzers/subset_font_fuzzer.cc",
+  ]
+  deps = [
+    ":sfntly",
+    "//base/test:test_support",
+  ]
+  seed_corpus = "fuzzers/corpora/sfntly_fuzzer"
+}
+
 if (build_sfntly_samples) {
   source_set("sfntly_subtly") {
     sources = [