Move GN build file for gmock_mutant from build/secondary into the main tree.

build/secondary/testing/BUILD.gn

Index: build/secondary/testing/BUILD.gn
diff --git a/build/secondary/testing/BUILD.gn b/build/secondary/testing/BUILD.gn
deleted file mode 100644
index fc50637493a77acf10b4f0855e2a2331a11b8031..0000000000000000000000000000000000000000
--- a/build/secondary/testing/BUILD.gn
+++ /dev/null
@@ -1,13 +0,0 @@
-# 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.
-
-source_set("gmock_mutant") {
-  sources = [
-    "gmock_mutant.h",  # gMock helpers
-  ]
-
-  deps = [
-    "//base",
-  ]
-}