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" ]
+
+  # TODO: Is the "libwebm_nacl" target used anywhere? If so, implement it in GN

[brettw, 2014/05/05 17:43:05]

I think you can remove this comment. With GN toolchain support we wouldn't have
a separate target name for the nacl one, just blocks to set the nacl-specific
flags:
  if (is_nacl) ... 
I don't see anything different in the nacl gyp file, so I think NaCl should
"just work" for this target.

[ckocagil, 2014/05/06 04:57:45]

I don't really understand. Doesn't nacl have its own toolchain and output
format? So, does it not need toolchain support and another target (like
libwebm_nacl in GYP)?

[brettw, 2014/05/14 16:40:14]

In GN you don't need a separate target to compile for a different toolchain. The
nacl build of libwebm would run this file and it gets output to a parallel
directory tree. If there are nacl-specific conditions, you can just write if
(is_nacl) inside this file.

See the toolchains section of https://code.google.com/p/chromium/wiki/GNLanguage
more

But the short answer is to just delete this comment.

+  # after GN gets nacl target capability.
+}