Add American Fuzzy Lop (afl) to third_party/afl/

third_party/afl/BUILD.gn

+# Copyright 2016 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.
+
+source_set("afl_runtime") {
+  # AFL needs this flag to be built with -Werror. This is because it uses u8*
+  # and char* types interchangeably in its source code. The AFL Makefiles use
+  # this flag.
+  cflags = [ "-Wno-pointer-sign" ]
+
+  configs -= [
+    # These functions should not be compiled with sanitizers since they
+    # are used by the sanitizers.
+    "//build/config/sanitizers:default_sanitizer_flags",
+
+    # Every function in this library should have "default" visibility.
+    # Thus we turn off flags which make visibility "hidden" for functions
+    # that do not specify visibility.
+    # The functions in this library will not conflict with others elsewhere
+    # because they begin with a double underscore and/or are static.
+    "//build/config/gcc:symbol_visibility_hidden",
+  ]
+
+  sources = [
+    "src/llvm_mode/afl-llvm-rt.o.c",
+  ]
+}