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

There is no need to keep it in the secondary tree.

BUG=None
TEST=gn gen out-gn/Debug
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1123183004

Cr-Original-Commit-Position: refs/heads/master@{#329102}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 16f0a88eea42d7528eaa71533e1c539c0bd45962

secondary/testing/BUILD.gn

@@ -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",
-  ]
-}