[gn] Add file needed for gn on mac to run

BUG=chromium:474921,chromium:626064

Review-Url: https://codereview.chromium.org/2134223003
Cr-Commit-Position: refs/heads/master@{#37679}

build_overrides/build.gni

+# Copyright 2016 The V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+mac_sdk_min_build_override = "10.10"
+mac_deployment_target_build_override = "10.7"
+
+# Variable that can be used to support multiple build scenarios, like having
+# Chromium specific targets in a client project's GN file etc.
+build_with_chromium = false
+
+# Uncomment these to specify a different NDK location and version in
+# non-Chromium builds.
+# default_android_ndk_root = "//third_party/android_tools/ndk"
+# default_android_ndk_version = "r10e"
+
+# Some non-Chromium builds don't support building java targets.
+enable_java_templates = true