Add GN targets for third_party/libwebm

third_party/libwebm/BUILD.gn

+# Copyright (c) 2014 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("libwebm") {
+  sources = [
+    "source/mkvmuxer.cpp",
+    "source/mkvmuxerutil.cpp",
+    "source/mkvwriter.cpp",
+  ]
+
+  # mkvmuxerutil.cpp redefines the _CRT_RAND_S macro. This is the only thing
+  # that blocks this target from using Chromium compiler settings.
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [ "//build/config/compiler:no_chromium_code" ]
+}