unbundle: add GN files for flac, harfbuzz-ng, and libwebp

build/linux/unbundle/flac.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.
+
+import("//build/config/linux/pkg_config.gni")
+import("//build/shim_headers.gni")
+
+pkg_config("system_flac") {
+  packages = [ "flac" ]
+}
+
+shim_headers("flac_shim") {
+  root_path = "include"
+  headers = [
+    "FLAC/all.h",
+    "FLAC/assert.h",
+    "FLAC/callback.h",
+    "FLAC/export.h",
+    "FLAC/format.h",
+    "FLAC/metadata.h",
+    "FLAC/ordinals.h",
+    "FLAC/stream_decoder.h",
+    "FLAC/stream_encoder.h",
+  ]
+}
+
+source_set("flac") {
+  deps = [
+    ":flac_shim",
+  ]
+  public_configs = [ ":system_flac" ]
+}