Port use_lld flag from gyp to gn.

build/toolchain/lld.gni

Index: build/toolchain/lld.gni
diff --git a/mojo/public/mojo_constants.gni b/build/toolchain/lld.gni
similarity index 56%
copy from mojo/public/mojo_constants.gni
copy to build/toolchain/lld.gni
index 3165a8dc9e2b7d223b1b3e2e9fe25ac30bd1c390..0853b481139392cc995826c3247ed234ec0009a9 100644
--- a/mojo/public/mojo_constants.gni
+++ b/build/toolchain/lld.gni
@@ -1,8 +1,11 @@
 # 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.
+# found in the LICENSE file.
+
+# Configures if the lld linker should be used.
 
 declare_args() {
-  # Mojo application directories are created within this subdirectory.
-  mojo_application_subdir = "Mojo Applications"
+  # Set to true to use lld.

[brettw, 2016/04/12 21:54:51]

Can you say what lld is? I didn't know and had to look it up.
Just like
  Set to true to use lld, the LLVM linker.

+  # Currently only has an effect on Windows.
+  use_lld = false

[brettw, 2016/04/12 21:54:51]

Will you need this outside of the windows toolchain setup? If not or if you're
unsure, I think this declare_args should just be moved to the top of that file.
We can always move it out later if we need to share it.

 }