Config changes to support target_os="fuchsia"

build/config/fuchsia/config.gni

Index: build/config/fuchsia/config.gni
diff --git a/build/config/fuchsia/config.gni b/build/config/fuchsia/config.gni
new file mode 100644
index 0000000000000000000000000000000000000000..d1d11048175263e882aed1edfe02d3977bbc789d
--- /dev/null
+++ b/build/config/fuchsia/config.gni
@@ -0,0 +1,10 @@
+# Copyright 2017 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.
+
+assert(current_os == "fuchsia")
+
+declare_args() {
+  # Path to Fuchsia SDK.
+  fuchsia_sdk = "//third_party/fuchsia-sdk"
+}