Linux: move use_system_harfbuzz logic out of the main file.

build/linux/unbundle/harfbuzz.gyp

+# Copyright 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.
+
+{
+  'targets': [
+    {
+      'target_name': 'harfbuzz-ng',
+      'type': 'none',
+      'cflags': [
+        '<!@(pkg-config --cflags harfbuzz)',
+      ],
+      'direct_dependent_settings': {
+        'cflags': [
+          '<!@(pkg-config --cflags harfbuzz)',
+        ],
+      },
+      'link_settings': {
+        'ldflags': [
+          '<!@(pkg-config --libs-only-L --libs-only-other harfbuzz)',
+        ],
+        'libraries': [
+          '<!@(pkg-config --libs-only-l harfbuzz)',
+        ],
+      },
+    },
+  ],
+}