Hook up WebKit WTF to GN build.

This also moves the "use clang plugins" flag to a GNI file so it can be shared (WebKit needs to read this flag).

TBR=scottmg

Review URL: https://codereview.chromium.org/248323004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@265940 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/clang/BUILD.gn

@@ -2,11 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-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 = true
-}
+import("clang.gni")
 
 config("find_bad_constructs") {
   if (clang_use_chrome_plugins) {

config/clang/clang.gni

@@ -0,0 +1,9 @@
+# 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.
+
+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
+}