Use proper clang plugin name in GN build.

build/config/clang/BUILD.gn

 # Copyright (c) 2013 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.
 
 config("find_bad_constructs") {
   cflags = [
     "-Xclang", "-load",
     "-Xclang",
+  ]
 
-    # TODO(brettw) express this in terms of a relative dir from the output.
-    # for now, assume the output dir is two levels deep under the source
-    # (like "out/Debug").
-    "../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
+  if (is_mac) {
+    cflags += rebase_path(
+      "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib",
+      ".", root_build_dir)
+  } else if (is_linux) {
+    cflags += rebase_path(
+      "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so",
+      ".", root_build_dir)
+  }
 
+  cflags += [
     "-Xclang", "-add-plugin",
     "-Xclang", "find-bad-constructs",
   ]
 }