[Extensions] Add fuzzer for Extension::ParsePEMKeyBytes()

Bug: NONE
Change-Id: I30b44520dbd7e69b7ed076afbed964f50e77455a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867123
Commit-Queue: Tim Judkins <tjudkins@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707489}

extensions/common/BUILD.gn

@@ -7,6 +7,7 @@ import("//build/config/jumbo.gni")
 import("//components/nacl/features.gni")
 import("//extensions/buildflags/buildflags.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//testing/libfuzzer/fuzzer_test.gni")
 
 # TODO(crbug.com/731689): Assert that extensions are enabled.
 
@@ -484,4 +485,14 @@ if (enable_extensions) {
       "//tools/metrics/histograms/enums.xml",
     ]
   }
+
+  fuzzer_test("extension_fuzzer") {
+    sources = [
+      "extension_fuzzer.cc",
+    ]
+    deps = [
+      ":common",
+      "//base",
+    ]
+  }
 }  # enable_extensions

extensions/common/extension_fuzzer.cc

+// Copyright 2019 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.
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "extensions/common/extension.h"
+
+namespace extensions {
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  std::string key_contents(reinterpret_cast<const char*>(data), size);
+  std::string key_bytes;
+  Extension::ParsePEMKeyBytes(key_contents, &key_bytes);
+
+  return 0;
+}
+
+}  // namespace extensions