Pull in libFuzzer as a third party dep.

third_party/libFuzzer/BUILD.gn

Index: third_party/libFuzzer/BUILD.gn
diff --git a/third_party/libFuzzer/BUILD.gn b/third_party/libFuzzer/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..7cb2ce96faf8ee5ccd050f7615866761eba566ad
--- /dev/null
+++ b/third_party/libFuzzer/BUILD.gn
@@ -0,0 +1,19 @@
+# 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("libfuzzer") {
+  sources = [
+    "src/FuzzerCrossOver.cpp",
+    "src/FuzzerDriver.cpp",
+    "src/FuzzerIO.cpp",
+    "src/FuzzerInterface.cpp",
+    "src/FuzzerLoop.cpp",
+    "src/FuzzerMain.cpp",
+    "src/FuzzerMutate.cpp",
+    "src/FuzzerSHA1.cpp",
+    "src/FuzzerTracePC.cpp",
+    "src/FuzzerTraceState.cpp",
+    "src/FuzzerUtil.cpp",
+  ]
+}