Add gn Blink GC plugin options

build/config/clang/clang.gni

 # Copyright 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.
 
 import("//build/toolchain/toolchain.gni")
 
 declare_args() {
   # Indicates if the build should use the Chrome-specific plugins for enforcing
   # coding guidelines, etc. Only used when compiling with Clang.
   clang_use_chrome_plugins = is_clang && !is_nacl && !use_xcode_clang
+
+  # Local clang plugin installation directory, re-configurable to accommodate
+  # distributed compilation setups.

[Nico, 2016/06/22 14:31:47]

Is it useful to switch plugin path independent from compiler path though? I'd
expect that if you set this, the plugin will just crash if it runs with the
pinned compiler.

(If this is useful to you, I'm not opposed to it, but I don't understand how it
can be useful yet.)

[sof, 2016/06/22 14:39:49]

I'm not using it, just carrying over pre-existing functionality.

But if you arrange for a suitable PATH, it should work.

+  clang_plugin_lib_path = "//third_party/llvm-build/Release+Asserts/lib"
 }